feat!: Narrow `seo` field types to their implementations.

[justlevine]

What

This PR improves the GraphQL schema by narrowing the NodeWithSeo.seo field type down to the implementation type (e.g. RankMathContentNodeSeo, RankMathPostSeo RankMathUserSeo etc.

That means instead of wrapping our queries with ...on *SEO { }, we can access the fields directly on the schema.

Why

Closes #79

How

Introduced a new Utils::overload_graphql_field_type() to get around https://github.com/wp-graphql/wp-graphql/issues/3096

Testing Instructions

Additional Info

Checklist:

• [x] My code is tested to the best of my abilities.
• [x] My code follows the WordPress Coding Standards.
• [x] My code has proper inline documentation.
• [x] I have added unit tests to verify the code works as intended.
• [x] The changes in this PR have been noted in CHANGELOG.md