Enable terraform-plugin-mux for Framework base support in 4.0

[jackofallops]

• Introduces provider-mux to pave the way for terraform-plugin-framework adoption post v4.0
• Adds 2 new provider functions

Community Note

• Please vote on this PR by adding a 👍 reaction to the original PR to help the community and maintainers prioritize for review
• Please do not leave "+1" or "me too" comments, they generate extra noise for PR followers and do not help prioritize for review

Description

This PR presents the first stage of support for terraform-provider-framework by introducing the mux server.

Use of the mux also allows for the addition of Provider Functions, a prototype function is included.

NOTE: This is WIP PR to facilitate testing across a wide selection of services and resources in CI and is no part is guaranteed to be taken forward and/or merged.

PR Checklist

• I have followed the guidelines in our Contributing Documentation.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have checked if my changes close any open issues. If so please include appropriate closing keywords below.
• I have updated/added Documentation as required written in a helpful and kind way to assist users that may be unfamiliar with the resource / data source.
• I have used a meaningful PR title to help maintainers and other users understand this change and help prevent duplicate work.
  For example: “resource_name_here - description of change e.g. adding property new_property_name_here”

Changes to existing Resource / Data Source

• I have added an explanation of what my changes do and why I'd like you to include them (This may be covered by linking to an issue above, but may benefit from additional explanation).
• I have written new tests for my resource or datasource changes & updated any relevent documentation.
• I have successfully run tests with my changes locally. If not, please provide details on testing challenges that prevented you running the tests.
• (For changes that include a state migration only). I have manually tested the migration path between relevant versions of the provider.

Testing

• My submission includes Test coverage as described in the Contribution Guide and the tests pass. (if this is not possible for any reason, please include details of why you did or could not add test coverage)

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

[katbyte]

we should add a comment around how we handle the metadatahost and env selection process here?

Discussed internally, review now out of date

[manicminer]

Thanks @jackofallops, a few minor suggestions but otherwise looks great 🚀

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.