Feat/new client with headers

[elizabethengelman]

What

Adds the ability to pass in headers to the HTTP client that is created in this client.

Companion cli PR: stellar/stellar-cli#1618

Why

Closes stellar/stellar-cli#1583

In order to allow for passing in headers to rpc providers, we need to build the HTTP client used in this client to include those headers.

Known limitations

N/A

[leighmcculloch]

One ask, otherwise I think this is good as is.

[elizabethengelman]

@leighmcculloch

It seems that the main branch of this repo is incompatible with main on the CLI because of the xdr update.

Should we wait until protocol 22 to merge in the accompanying CLI PR? Or, does it make sense to have this PR target v21.4.0 and release 21.4.1 here in the rpc client instead so we can get this API key support out there sooner?

[leighmcculloch]

@janewang What's the urgency on API support?

If we can delay on API support, I'd rather keep backports at this point to patch releases fixing bugs, rather than new features, just to reduce the divergence in code that we have to manage. But if API key support is urgent then lets target v21 here, and we can backport the CLI change.

[janewang]

@leighmcculloch The API support change can wait for v22. It's asked by the community but I think it could wait for a few weeks

[elizabethengelman]

@leighmcculloch @janewang, sounds good to delay support for this until v22. I'll make sure to keep this branch up-to-date with main in the meantime.

[elizabethengelman]

Actually, I suppose that means we can merge this in now - does that sound right @leighmcculloch ?