Support for other methods in subscribe (other than eth_subscribe)

[danielharrisgsr]

Is your feature request related to a problem? Please describe.
For example, with the Reth client it is possible to define custom namespaces for extending functionality, such as adding RPC and subscription methods. Some call with method eth_subscribe and params ["newHeads"] might have a custom implementation (for some reason) in a custom namespace with method my_eth_subscribe and params ["myNewHeads"]

The current implementations of the JsonRpcClient trait bakes in the "eth_subscribe" string in a number of places, which makes it unable to support calling of these custom methods

• subscribe() will call eth_subscribe under the hood. This can be avoided by making your own request() and passing the channel_id into a SubscriptionStream, although this is more fiddly...
• ... and the above is actually not possible because the TransactionManager will only add a channel_id to its channel map field if the method is eth_subscribe. So attempting to listen to the stream you requested will error
• reconnect() method also hardcodes the eth_subscribe method for its attempted reconnection to a stream

Describe the solution you'd like
Support for additional methods in the subscription case, through careful removal of reliance on the string "eth_subscribe"

Describe alternatives you've considered

• Current workaround is to simply use the eth namespace in extending Reth, which allows the ethers calls to work
• Additionally, one can not use ethers and instead roll their own websocket connection capabilities with tungstenite or other

Additional context