Represent TerraformBackend.Config with cty.Value

[mr0re1]

• Add Dict struct to map string ket to cty.Value and handle YAML marshal/unmarshal;
• Use Dict instead of map[string]interface{} for TerraformBackend.Config.

Submission Checklist

• Have you installed and run this change against pre-commit? (pre-commit install)
• Are all tests passing? (make tests)
• Have you written unit tests to cover this change?
• Is unit test coverage still above 80%?
• Have you updated all applicable documentation?
• Have you followed the guidelines in our Contributing document?

[tpdownes]

• fix minor typos
• address concern about old library (if there is no alternative please confirm that this isn't tying us to old packages or old version of Golang)
• minor Qs about parsing

May not require code changes other than typos.