Introduce tfjsonpath Package and Sensitive and Unknown Plan Checks

[SBGoods]

This PR implements the design described in [TF-727] Terraform Plugin Testing Sensitive and Unknown Plan Checks and adds the following:

• tfjsonpath package which defines methods that allows provider developers to traverse Terraform JSON data
• ExpectUnknownValue() plan check which asserts that the specified attribute is unknown in the plan
• ExpectSensitiveValue() plan check which asserts that the specified attribute is marked sensitive in the plan

[bendbennett]

Great job!

Left a couple of minor comments.

The documentation is superb. I particularly like the the methodical 1-2-1 comparison of SDKv2 and framework syntax. I'me wondering whether the schema structures that are described in this context are more broadly useful in terms of providing "templates" for provider developers that are migrating from SDKv2 to the framework?

[bendbennett]

Looks really great 🚀

[bflad]

Overall this looks really good, optimistically approving barring some of the below suggestions. 🚀

[github-actions]

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