Add support to query supported languages (with installed drivers)

[smola]

In some applications, it is very useful to know which languages are supported (with installed drivers) beforehand. This helps to:

• Display lists of supported languages in UX elements.
• Reduce number of requests to bblfshd by avoiding requests for unsupported languages.

It would be nice to have such functionality implemented in the Scala client.

[dpordomingo]

We have that necessity for bblfsh/web#89 too.
It was proposed to use the github.com/bblfsh/bblfshd/daemon/protocol.ProtocolServiceClient, but it currently runs over a unix socket so it is not reachable from the outside of the bblfsh container.

[bzz]

Indeed we need this, but for golang client first :)

This makes me think - do you guys @juanjux @dennwc think this is the right repo to track this, or is there a better one if changes are required on a protocol-level? If not, I guess we should at least add similar one to https://github.com/bblfsh/client-go/issues

[juanjux]

Should probably be in the SDK since the protocol packages are there.

[juanjux]

Closing since we just got a duplicated in the SDK

sdk/issues/253

[juanjux]

Still needs to be integrated in the client, reopening.