Support multiple signers loaders

[ltitanb]

We currently support two loaders for the signer module:

commit-boost-client/crates/common/src/loader.rs

Lines 18 to 27 in e6ad292

	pub enum SignerLoader {
	/// Plain text, do not use in prod
	File {
	key_path: String,
	},
	ValidatorsDir {
	keys_path: String,
	secrets_path: String,
	},
	}

In particular the ValidatorsDir supports the ERC-2335 keystore format as used in Lighthouse.

We should:

• add support for other keystores as used in other CL clients
• add support for remote signers, such as web3signer and Dirk

We can open sub-issues for each one as needed

[DrewVanderWerff]

Should this be closed?