refactor(resolve): change FetchSchema to support custom schema loaders

[bmfs]

Added support for custom schema loaders based on the URI scheme

Some things to highlight:

• Created a global loader registry similarly to the schemaRegistry and keywordRegistry
• Converted http(s) and file to loader functions and they included by default on the global registry.

[bmfs]

@Arqu let me know if anything is missing in the PR

[Arqu]

@bmfs thank you for the work on this, this is lovely, and cracks open some other issues that are outstanding.

Before merging, I'd require you to add some comments to respect the linter go lint and have everything pass there.

Specifically:

schema_loader.go:14:6: exported type LoaderRegistry should have comment or be unexported
schema_loader.go:18:6: exported type SchemaLoaderFunc should have comment or be unexported
schema_loader.go:20:1: exported function NewLoaderRegistry should have comment or be unexported

Also a nitpick I do have is the GetGlobalLoaderRegistry function naming, would rather it be just GetSchemaLoaderRegistry or GetLoaderRegistry, but if you feel strongly about highlighting this is specifically for the global registry, it's fine for now. In a future update where the thread safety is addressed this will probably be absorbed and changed.

Other than that, happy to merge as is and especially glad with the added flexibility.

[Arqu]

LGTM, thank you!