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

[elizabethengelman]

In Slack, we talked about creating an init fn that was more future-proof than this, that allowed for passing in a function that could customize the client as it was being created. The issues I'm running into with that are:

1. The http_client field on this rpc Client is private and so we cannot get or set it from outside of this file. This is a pretty straightforward fix in that we could make that field public, or create a setter method. But do we want to expose the http_client outside of the rpc-client?
2. The headers field on the HttpClient (created by jsonrpsee crate) is also private, so we wouldn't be able to just update the headers on the http_client, but instead need to rebuild a new http_client.

If we did create an init fn that took a fn as an arg, it would look something like the following. We'd also need to add http_client_setter:

    pub fn set_http_client(&mut self, http_client: HttpClient) {
        self.http_client = Arc::new(http_client);
    }

    pub fn init<F>(base_url: &str, f: F) -> Result<Self, Error>
    where
        F: FnOnce(Self) -> Self,
    {
        let client = Self::new(base_url)?;
        Ok(f(client))
    }

And then the cli code would end up looking something like this...

 pub fn new(network: Network) -> Result<Self, Error> {

       // get the additional headers from the network config

        let customize =  |mut client: rpc::Client| -> rpc::Client {
            let base_url = client.base_url();
            let mut headers = rpc::Client::default_http_headers().clone();

            for (key, value) in additional_headers {
                headers.insert(key.unwrap(), value);
            }

            let http_client = HttpClientBuilder::default()
                    .set_headers(headers)
                    .build(&base_url).unwrap();
            client.set_http_client(http_client);

            client
       };
       
        let client = rpc::Client::init(&network.rpc_url, customize)?;
      
       Ok(Self { client })
}

I kind of feel like this solution would bit more complicated than what is in the PR already, while still only allowing us to customize the HTTP client. I'm also not sure we want to expose the HTTP client outside of the rpc-client. But I'm curious to hear what others think!