Create RPC manifest

[mycognosist]

It seems useful at this point to have a list of RPCs supported by kuska (separated by request and response).

I intend on generating the list and including the shape in which data is expected for each method.

[decentral1se]

Might be useful to see that go-ssb simply provides a JSON response which is hard-coded:

https://github.com/ssbc/go-ssb/blob/3d0822793d8ba3c2e2166e0f58594a048860052e/sbot/manifest.go#L60-L149

[mycognosist]

Yeah maybe a simple manifest.json is a reasonable approach for the time being.