[pulsaradmin] Add GetAllSchemas command

[calindima]

Didn't create an issue but I can if that's necessary, otherwise I'll write the details in this PR.

Motivation

Explain here the context, and why you're making that change. What is the problem you're trying to solve.

We actively use pulsarctl which has a dependency on this library. We have a use case where we want to interogate the SchemaRegistry to retrieve a history of schema versions and observe schema evolution for topics with higher retention (e.g. infinite retention, >6mo retention etc.).

Modifications

Describe the modifications you've done.

The PR only contains additions such as:

• Added GetAllSchemas method to the Schema interface
• Added the implementation for the GetAllSchemas method
• Added a model for the Pulsar Admin API response
• Added a utility method to convert from this model to a list of SchemaInfoWithVersion

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Added an integration test that uploads a schema on a topic and then asserts the length of all schemas is 1

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (yes)
• The schema: (don't know)
• The default values of configurations: (no)
• The wire protocol: (no)

Documentation

• Does this pull request introduce a new feature? (yes)
• If yes, how is the feature documented? (doc comment)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a followup issue for adding the documentation

Just let me know if there's any documentation I should update - I only added a godoc comment to the new method

[calindima]

Thanks for running the workflows @crossoverJie. I fixed the linter error but the failed integration test is not related to anything changed in this PR as far as I can tell.

[crossoverJie]

Thanks for running the workflows @crossoverJie. I fixed the linter error but the failed integration test is not related to anything changed in this PR as far as I can tell.

Yes, I have approved it. Thank you for your contribution

[calindima]

Thanks for the quick feedback. Any idea in which release will this be included?