Enabling custom header injection for Clients

[desimmons]

Would maintainers of this library accept custom header injection functionality for clients? This would enable using nodes linked to Dhali, which require the custom Payment-Claim header to be used. I was thinking of including an optional constructor arg get_custom_headers, something like this:

from xrpl.clients import JsonRpcClient

def get_custom_headers():
    return {"Payment-Claim": "<a payment claim>"}

dhali_linked_rpc_node = "<url of dhali linked rpc node>"

client = JsonRpcClient(dhali_linked_rpc_node, get_custom_headers=get_custom_headers)

[mvadari]

Related: #763