terraform test: Fill computed values of Nested collection type

[dsa0x]

Fixes #37939

This PR enables the mocking framework to fill in mocked values of collection types (set, list, map). The issue particularly affected providers that implement these collection types as a NestedBlock-style object, a behaviour which terraform knows how to deal with. In the PR, we can see that very little code change was made, because the ConfigType() method on the schema object has already helped us in converting the different block types into valid cty Types, e.g NestingList => cty.List. The returned type can then be used with fillType, which essentially traces all paths, and ensures that the mocked value adheres to the schema, or generates a value for the path.

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.