Support capabilities when using the HTTP API

[milesj]

While capabilities are a gRPC only feature, it would be nice if the HTTP API also supported returning what capabilities are available, probably under a /capabilities route.

Right now, we have no way of knowing what features downstream users are using. Do they have zstd enabled? What's their max upload size? So on and so forth.

/status doesn't provide all the information that we need.

[mostynb]

Hi, is there a specific list of settings you need to be able to query?

[milesj]

The primary settings are what digest functions and compressors can be used.

The other capabilities I can set to defaults or force enable them (like cas/ac stuff) since it's always enabled.