Add more metadata to `available-connectors`

Extend the metadata provided by `Workspace.listAvailableConnectors`:

 - the npm package (name, version spec) - this workspace to automatically add a dependency record to project's package.json
 - features - this is used by Studio to decide whether "discovery" and "autoupdate" is available
 - connector settings - this can be used by both Studio and `yo loopback:datasource` to build user interface for editing basic connector configuration

Also add `mail` connector.

/to @ritch please review
/to @raymondfeng please review that I have correctly filled the information about which connectors support discovery and automigration.