support querying the server for installed drivers and supported drivers about bblfshd

[abeaumont]

As discussed with @mcuadros, we will add support both for server version and installed drivers, in the following way:

• The server will offer 3 new request types:
  • version. The response will provide server version information, which may include the commit hash and/or a tag.
  • get_driver. The request will include the language name. The response will provide the manifest of the driver, which the server will retrieve from the file-system.
  • get_drivers. The response will provide a list of installed drivers, with its language, version and image_id

[vmarkovtsev]

As for now, hardcode! 😆 https://github.com/src-d/ast2vec/blob/master/ast2vec/repo2nbow.py#L51

[vmarkovtsev]

One of the possible implementations is to query a fake file for every language (the list can be taken from src-d/enry or github/linguist), record failures and build the registry. Can take some time if implemented straightforward; I bet there is a docker API to check the existence of a container very fast.

[smola]

We should also expose which version of the driver is installed.

[vmarkovtsev]

Any updates?

[abeaumont]

No updates, it's something we want to do at some point, but are focused on other tasks at the moment.

[abeaumont]

The listing of supported drivers may be tricky, since the server may support custom images for some languages, and building a list of images through a complete list of languages and filtering sounds rather innefficient.
A hardcoded (but configurable) list of supported drivers, as suggested by @smola, is probably more efficient. In this case, it should probably be related with #80.

In any case, we can start adding support for installed drivers, to begin with.