Allow CustomSchemaProvider to provide schema content instead of uri

[qvalentin]

Is your enhancement related to a problem? Please describe.

A Lsp Client can implement custom schema selection by using the CustomSchemaProvider.
This only allows the client to return a uri to the schema to be used.
For my usecase the client is generating the schema on the fly.
Only being able to return a uri adds an additional roundtrip that could be avoided.

Describe the solution you would like

Allow the CustomSchemaProvider to directly return the schema as json.

Describe alternatives you have considered

Additional context