Simplify schema provider definition

[tbenr]

Removes the need to specify constant or variable schema provider when defining them via builder.
With this PR the non-changing instance across multiple forks is automatically guaranteed via an equality check.

This approach provide a nice benefit: you don't need to care about if the schema is changing or not. If the shema is an "high order" schema that looks up in the registry to pick the schemas for the fields, you don't know if those fields are changing across forks. This way we can be sure that we will always keep the new one whenever the new schema is semantically different from the previous one.

There are some edge cases in which the equality check may not do what we want (maybe?) so I provided a way to disable it.

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[zilm13]

LGTM just few nits