Fix Signer implementation

[thomas-senechal]

Context
The current implementation of Signer is broken. It is supposed to allow us to do multiple implementations of signers to be able to sign, for example, using Massa Wallet, a Secret Key from env etc. We only used it for MW in MS, and overtime, things got mixed up and the implementation is now tied to MW.

We need to modify the implementation to allow more modularity and ensure that any kind of Signer implementation is possible.

How to

• Move WalletPlugin signer to it's own package
• Move https://github.com/massalabs/station/blob/9abe68848e10c65211982901d6a5ad647bef4404/pkg/node/sendoperation/sendoperation.go#L81C1-L83C52 to the WalletPlugin signer
• Ensure everything is working fine

QA testing

• Download MS
• Install MS
• Install MW
• Try to do some operations on buildnet
• Everything should work fine