Adding core.Keystore to LOOP Initialization

[patrickhuie19]

Note that this PR originally introduced a CSASigner type. https://github.com/smartcontractkit/chainlink-common/pull/1050 ran into a similar need to hide a Ed25519 private key behind an interface to aid with the EVM extraction. We will pass a core.Keystore type around to LOOPs, but call it a CSASigner to make it explicit.

Requires

Supports

CsaAuth service in Beholder

[jmank88]

The CSA key used in the EVM relayer gets passed through to wsrpc for x509 cert generation, and the underlying sign methods uses the crypto.SignerOpts type. It is open ended but we may need to support a uint hash type, and string context for the signing on the other end to use.

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