Add support for KMS and keystore signers in Relay

[onbjerg]

Extend the current signer flags to accept different kinds of signers, like AWS KMS and keystore signers.

Note that these flags should be mutually exclusive, e.g. if I pass --secret-key, I cannot also enable AWS KMS.

Depends on #90

[gakonst]

• https://docs.rs/alloy/latest/alloy/signers/gcp/struct.GcpSigner.html
• https://docs.rs/alloy/latest/alloy/signers/aws/struct.AwsSigner.html
• https://docs.rs/alloy/latest/alloy/signers/aws/struct.AwsSigner.html#impl-TxSigner%3CPrimitiveSignature%3E-for-AwsSigner
• https://docs.rs/alloy/latest/alloy/signers/aws/struct.AwsSigner.html#impl-Signer-for-AwsSigner
• https://docs.rs/alloy/latest/alloy/network/trait.TxSigner.html
• https://docs.rs/alloy/latest/alloy/signers/trait.Signer.html

just dumping these for vis

[aroralanuk]

If this is open to external contributors, can I pick this up?

[onbjerg]

Sure! Some inspiration here: https://github.com/init4tech/builder/blob/375d976c112f0a7e6de0d4c9790166805bcc1e92/src/signer.rs#L52