Add support for workspace/configuration requests

[alcarney]

• Using the new client.set_configuration method, it's now possible to set configuration values at various scopes and configuration sections

  global_config = {"values": {"a": 42, "c": 4}}
  
  workspace_uri = "file://workspace/file.txt"
  workspace_config = {"a": 1, "c": 1}
  
  client.set_configuration(global_config)
  client.set_configuration(
      workspace_config, section="values", scope_uri=workspace_uri
  )

  If the server makes a workspace/configuration request during a test case the client will automatically respond accordingly

• pytest_lsp will emit a warning if the capabilities used by the client do not indicate support for workspace/configuration requests.

Closes #90