Extract cnrc into separate project

[tzdybal]

#399 introduces simple library to handle celestia-node RPC endpoints.

In my opinion, this library can be useful on it's own. My idea is:

1. extract this into separate project (name proposals are welcome!)
2. prepare testing environment (testing unit tests with ephemeral-cluster)
3. invite community to implement missing endpoints - create issues for all missing features

[adlerjohn]

Not a bad idea. Would also make it easier to implement in different languages.

[jbowen93]

I definitely think it should be a separate library. celestia-node-client.

It's important to note this isn't a RPC client. This is a REST client.

[Wondertan]

I like the idea of extracting alpha API into a separate repo, so we can easily deprecate it once the time comes. Surely, there are also other good reasons to extract it.

Note that future Public API in celestia-node inherently covers the client-side of the communication. Ideally, Public API would be a set of interfaces over each celestia node module with two implementations: one over an embedded node and another over an RPC client that talks to a remote Node. The latter will supersede the Alpha API eventually, and these interfaces would live in the celestia node repo or extract to a separate API repo, which is TBD.