add neuron certificate handling

[andreea-popescu-reef]

Description

implemented logic for neuron certificate discovery and storage:

• add neuronInfo_getNeuronCertificate: ( netuid, hotkey ) --> certificate pallet storage for neuron certificates
• add fn get_neuron_certificate(netuid, neuron_uid) rpc to be called via bittensor library to fetch the certificate for a particular neuron
• add fn serve_axon_tls(..., certificate) as an alternative for existing serve_axon taking an additional certificate parameter and storing it

compatible with older versions of bittensor as it doesn't modify the behaviour of existing rpc calls.

Checklist

• Added tests for intended behaviors
• Added tests for error and/or panic states
• Updated relevant documentation
• Tracked rustfmt changes
• No new compiler warnings

[emnoor-reef]

Could not compile due to certificate module non-existing. I could be wrong, but if a file was missing from git, I'll look at it again once you add the file.

I have little to no idea about subtensor internals, so left some style related nitpick comments 😁. But going through it, it looks good to me (assuming certificate::Certificate is just an alias for Vec<u8>), apart from the nomenclature thing pawel mentioned.

[emnoor-reef]

As odd pointed out in slack, having [u8; 32] would be more appropriate than Vec<u8> as the certificates are supposed to be 256-bit in length. We don't want to allow users to store data of arbitrary size here.

[emnoor-reef]

Here and every other method the returns a certificate should have a return type of Option<...> ( Option<Vec<u8>>, Option<[u8; 32]> if we go the array route).

I'd also prefer using Certificate instead of Vec<u8>/[u8; 32] as we are defining a type anyway.