feat: generate extension manifest schema

[ericswanson-dfinity]

Description

dfx schema --for extension-manifest now generates the JSON schema for extension.json files.

How Has This Been Tested?

Updated CI workflow

Checklist:

• [x] The title of this PR complies with Conventional Commits.
• [x] I have edited the CHANGELOG accordingly.
• [x] I have made corresponding changes to the documentation.