Use `solana_signature::Signature`

[emhane]

Use solana_signature::Signature instead of d25519_dalek::Signature

[448-OG]

You didn't give a reason for the switch? Also solana_signature::Signature uses ed25519-dalek under the hood so I am adding more dependencies to the core wallet-adapter library.

[emhane]

seamless integration with the rest of solana rust ecosystem, so don't get error

expected `solana_keypair::Signature`, found `wallet_adapter::ed25519_dalek::Signature`