Extract the new enum and union type extensions supported by graphql-js

[jansuchy]

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

Similarly to #948, this makes extractExtensionDefinitions understand the new union and enum type extensions

Please note that this PR does not make the enum and union schema extensions "just work" with
makeExecutableSchema and others, because these -- same as the input type extension added in #948 -- actually need graphql-js v14. But at least these definitions are not skipped and we get a good descriptive message what's wrong:

     Error: The UnionTypeExtension kind is not yet supported by extendSchema().

[mfix22]

@jansuchy this looks good to me. Could you update the CHANGELOG to add your commit to the current vNext instead?

[jansuchy]

Thanks for review @mfix22 Changelog updated.