feat: introduce appendix section

[Kavindu-Dodan]

This PR

Adds an appendix section [1]

Appendix is expected to contain extra information for both OpenFeature maintainers and users

[1] - open-feature/ofep#71 (comment)

[toddbaert]

Relative to most spec changes, this one seems low risk and high reward. I personally would love to remove the dependency cycle in our SDK repos that results from including the flagd provider. The OFEP was also well-received, so I'm going to merge this EOD unless I hear objections.

[federicobond]

Just to check, this would effectively deprecate the caching tests scenarios declared in the e2e test-harness when using the in-memory provider, right?

[toddbaert]

Just to check, this would effectively deprecate the caching tests scenarios declared in the e2e test-harness when using the in-memory provider, right?

Yes, good call. I think the caching tests are no longer relevant, and beyond the scope of what SDKs should be testing.

In fact, we may want to move the gherkin suite(s) into their own appendix in the spec and de-emphasize the test-harness repo, which is tightly associated with flagd.