Use the configured AWS partition when constructing ARNs

[mbbush]

Description of your changes

Stores the existing partition id from the provider config into a terraform configuration object, which is available when resolving external names.

Uses the partition as part of the external name config for the resources that currently hardcode a partition of aws in their templated ARN.

Refactors the external name configurations which create arns to use simple helper functions, to reduce code repetition.

The default value of the partition is "aws", which is currently the only supported value, so this should be a nonbreaking change for users.

Fixes #881 Fixes #820 Addresses #855 (although probably not completely)

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

[github-actions[bot]]

This provider repo does not have enough maintainers to address every pull request. Since there has been no activity in the last 90 days it is now marked as stale. It will be closed in 14 days if no further activity occurs. Leaving a comment starting with /fresh will mark this issue as not stale.

[github-actions[bot]]

This pull request is being closed since there has been no activity for 14 days since marking it as stale. If you're still working on this, feel free to reopen the PR or create a new one!