api: Add an optional requests retry feature to the RPC client

Add a new enable_requests_retry flag to the swh.core.api.RPCClient class, default to False, allowing to retry requests sent by the client raising connection errors and transient remote exceptions. Such failed requests will be retry at most five times with a delay of 10 seconds between each.

This should help mitigate the TransientRemoteException and ConnectionError exceptions raised by the scheduler RPC client.