Fill computed values of Nested 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. 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

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CHANGELOG entry

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• This change is not user-facing.