PWX-28191: Add support for reading AWS credentials from a k8s secret.

[adityadani]

What this PR does / why we need it:

• In PXOne, while installing Portworx on an EKS cluster, PXOne will create a kubernetes secret with the required AWS on the cluster in the same namespace where Portworx will be installed.
• This PR adds support for reading such AWS credentials from a kubernetes secret and creating an AWS static credentials provider. This static credentials provider is chained with our existing credentials providers - env variable provider & ec2 instance role provider.
• Added UTs to read secrets from a fake k8s clientset.

Which issue(s) this PR fixes (optional) Closes #

Special notes for your reviewer:

• Will add auto-refreshing mechanism for refreshing these static credentials in a subsequent PR.

Testing Notes:

• Tested w/o adding a kubernetes secret. The existing ec2 instance role way of authenticating succeeds.
• Disabled the ec2 instance role and added a kubernetes secret. Ensured PX installs successfully and pool expansion operations succeed.

[codecov-commenter]

Codecov Report

Patch coverage: 5.88% and project coverage change: -0.03% :warning:

Comparison is base (a6ffd89) 8.12% compared to head (e6796ad) 8.10%.

:exclamation: Current head e6796ad differs from pull request most recent head c5f597f. Consider uploading reports for the commit c5f597f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #151 +/- ## ========================================= - Coverage 8.12% 8.10% -0.03% ========================================= Files 18 18 Lines 4703 4716 +13 ========================================= Hits 382 382 - Misses 4299 4312 +13 Partials 22 22 ``` | [Files Changed](https://app.codecov.io/gh/libopenstorage/cloudops/pull/151?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libopenstorage) | Coverage Δ | | |---|---|---| | [aws/aws.go](https://app.codecov.io/gh/libopenstorage/cloudops/pull/151?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=libopenstorage#diff-YXdzL2F3cy5nbw==) | `7.56% <5.88%> (-0.14%)` | :arrow_down: |

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