Add support for deprecating variables and (module) outputs

[DanielMSchmidt]

This PR introduces a new attribute called deprecated to variable and output blocks. It is a string, that if set indicates that this variable / output should be treated as deprecated.

Deprecated variables emit a warning diagnostics if a value is set (be it at the root module level or as part of a module call).
Deprecated outputs emit a warning if they are referenced anywhere. Deprecated outputs can take deprecated values (where e.g. the deprecation comes from another output or from a deprecated resource attribute) without emitting a warning (so that if you are a module author and a module you depend on deprecates an output (or your module has a deprecated resource attribute) you forward you can "forward" the deprecation to the end-consumer of your module (and safely remove it in the next major release of your module). You can not set root module outputs as deprecated, you should be able to remove them safely already.

You will get also deprecation warnings for all illegal uses in modules you consume (as we don't know what is under your area of influence). If a module is outside your control you can use ignore_nested_deprecations to ignore any nested deprecation warnings. If the module emits a deprecated value you will still get a warning for that since it's within your control.

Fixes #5949

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[jbardin]

I think this should be broken up into 2 or 3 smaller PRs. The new concept of deprecation marks is definitely worthy of a standalone review. Resource attribute deprecation is an existing concept which could then make use of the new mechanism, while module output deprecation is an entirely new addition.

[jbardin]

I wonder if we might want some sort of path info here too, to indicate both the object and attribute path used. The Origin points to the actual source location, but doesn't directly tell the user what was deprecated, and the messages will often not state that explicitly.
Maybe we should add a globalref.Reference, which also includes the source range?