Add loopback-connector-openapi and loopback-connector-grpc to be part of the available connectors

[dhmlau]

Suggestion

Add loopback-connector-openapi and loopback-connector-grpc to be part of the available connectors.
The list is coming from https://github.com/strongloop/loopback-workspace/blob/master/available-connectors.json

Acceptance criteria

• when running lb4 datasource, the openapi and grpc connectors show up as one of the available connectors.
• update Connectors reference
  ( also remove Swagger connector https://loopback.io/doc/en/lb4/Swagger-connector.html once its doen)

cc @raymondfeng