Implement workspace/textDocumentContent for json schema retrieval in code lenses

[rwols]

Is your enhancement related to a problem? Please describe.

Currently, yaml-language-server has functionality that works as follows:

1. It shows a code lens at the top of a YAML file to view the schema.
2. The code lens contains a command (jumpToSchema).
3. When the client runs the command with workspace/executeCommand, yaml-language-server will in turn invoke window/showDocument.
4. The document to show has a URI with scheme json-schema.
5. The client has to have some glue code to handle that schema.

Describe the solution you would like

Implement workspace/textDocumentContent from the (new, as of this writing) 3.18 LSP spec. The flow would then look like this:

1. It shows a code lens at the top of a YAML file to view the schema.
2. The code lens contains a command (jumpToSchema).
3. When the client runs the command with workspace/executeCommand, yaml-language-server will in turn invoke window/showDocument.
4. The document to show has a URI with scheme json-schema.
5. The client sees that yaml-language-server has support for json-schema schemes
6. The client issues a workspace/textDocumentContent request to yaml-language-server
7. Yaml-language-server returns the content.
8. The client displays the content.

Describe alternatives you have considered

N/A

Additional context

N/A