Add a capabilities endpoint.

[claymcleod]

I suggest that we add an endpoint that informs clients of the capabilities of the execution service (similar to capabilities) in the LSP specification. For example, a client should be able to, at minimum, figure out the following:

• What is the total capacity a service can provide (including pending jobs, even if it is unbounded).
• What is the expected concurrent capacity that a service can provide (just running jobs, even if it is unbounded).
• Does the service support preemptible instances?

[vsmalladi]

Intersting idea. That could be under service_info

[patmagee]

@vsmalladi if you remember this was one of the outcomes from our discussion a while back as well

[uniqueg]

See #188 and especially ga4gh/TASC#45