RFC: Add "capabilities" section to version command output

[wez]

Keeping in mind our versioning and compatibility scheme, clients are better off interrogating a set of capabilities rather than performing version number checks.

I'm proposing that we augment the version command so that it outputs something like this:

{
    "version": "3.5.0",
    "capabilities": ["relative-roots"]
}

Including the capabilities list in the version command allows clients to query the version number and obtain the capability set in a single round trip. If we were to add a separate command just for capabilities, clients would need to query the version number to determine if they could then query for capabilities.

As part of this we should define the capabilities. I'm inclined to just have relative-roots as the first and only listed capability initially. All other functionality is effectively and implicitly available if the capabilities field is present in the version output.

I'd like to define relative-roots as complete support for watch-project with the relative_root parameter for the query, trigger and subscription subsystems.

An additional command that we can add would be to help check compatibility, so for example, the client would do:

["capabilities", {"required": ["relative-roots"], "optional": ["pcre"]}]

The response would look something like:

{
   "version": "3.7.0",
   "capabilities": {
       "relative-roots": true,
       "pcre": false
   }
}

If any of the capability names in the required field are missing, the capabilities command would add an error field listing the missing capabilities, causing the command to error out. The capabilities map has boolean values that indicate whether a given capability is supported.

Thoughts?