Add support for aks managed identities

[gshaw-pivotal]

1. What this PR does / why we need it:

Support managed identities specification when creating and updating AKS clusters

2. Which issue(s) this PR fixes

N/A

3. Additional information

When using terraform apply a user can provide a identity_config section within the cluster config section of the terraform plan file. Here they can specify whether a system or user assigned managed identity is to be used for the cluster.

The default if an identity_config is not provide will be use a system assigned managed identity.

4. Special notes for your reviewer

Changes were tested by creating and modifying clusters on Olympus dev stack. Scenarios tested are summarized below:

• No identity_config struct specified: SUCCESS -> Cluster with system assigned MI
• Empty identity_config struct: SUCCESS -> Cluster with system assigned MI -> Defaults to system assigned MI in terraform plan

• identity_config struct with type IDENTITY_TYPE_SYSTEM_ASSIGNED: SUCCESS -> Cluster with system assigned MI

• identity_config struct with type IDENTITY_TYPE_USER_ASSIGNED and no userAssigned struct: ERROR -> No cluster -> Expected behavior, see Terraform test - User assigned identity with no userAssigned struct attachment
• identity_config struct with type IDENTITY_TYPE_USER_ASSIGNED and empty userAssigned struct: ERROR -> No cluster -> Expected behavior, see Terraform test - User assigned identity with empty struct attachment
• identity_config struct with type IDENTITY_TYPE_USER_ASSIGNED and populated userAssigned struct, invalid identity: ERROR -> No cluster -> Expected behavior, see Terraform test - User assigned identity with invalid identity attachment

• identity_config struct with type IDENTITY_TYPE_USER_ASSIGNED and populated userAssigned struct, valid identity: SUCCESS -> Cluster with user assigned MI

• Create cluster with system assigned MI: SUCCESS
• Create cluster with user assigned MI: SUCCESS
• Create cluster with system assigned MI, then change to a user assigned MI: SUCCESS -> List aks clusters now shows user assigned identity for cluster
• Create cluster with system assigned MI, then change to a user assigned MI, then change back to a system assigned MI: SUCCESS -> List aks cluster shows system assigned identity again for cluster; SYSTEM -> USER -> SYSTEM
• Create cluster with user assigned MI, then change to a system assigned MI: SUCCESS -> List aks clusters now shows system assigned identity for cluster

• Create cluster with user assigned MI, then change to a system assigned MI, then change back to user assigned MI: SUCCESS -> List aks cluster shows user assigned identity again for cluster; USER -> SYSTEM -> USER

  Terraform test - User assigned identity with no userAssigned struct:

Terraform test - User assigned identity with empty struct

Terraform test - User assigned identity with invalid identity

[vmwclabot]

@gshaw-pivotal, you must sign every commit in this pull request acknowledging our Developer Certificate of Origin before your changes are merged. This can be done by adding Signed-off-by: John Doe <john.doe@email.org> to the last line of each Git commit message. The e-mail address used to sign must match the e-mail address of the Git author. Click here to view the Developer Certificate of Origin agreement.

• Commit 98f2b997457455a00ee943c899b3f9492afd53f1 must be signed by gshaw+github@pivotal.io
• Commit 45e5d8f83b9cb50694fe56f81a49d609419cc563 must be signed by gshaw+github@pivotal.io

[codecov-commenter]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (8c08715) 25.09% compared to head (92c4692) 25.24%.

Files	Patch %	Lines
internal/resources/akscluster/akscluster_mapper.go	75.51%	8 Missing and 4 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #359 +/- ## ========================================== + Coverage 25.09% 25.24% +0.14% ========================================== Files 191 191 Lines 16480 16529 +49 ========================================== + Hits 4135 4172 +37 - Misses 12131 12139 +8 - Partials 214 218 +4 ```

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[vmwclabot]

@gshaw-pivotal, you must sign every commit in this pull request acknowledging our Developer Certificate of Origin before your changes are merged. This can be done by adding Signed-off-by: John Doe <john.doe@email.org> to the last line of each Git commit message. The e-mail address used to sign must match the e-mail address of the Git author. Click here to view the Developer Certificate of Origin agreement.

• Commit 131aeee68688192ce127d517b423dc1a13c84380 must be signed by gshaw+github@pivotal.io