feat: support external signers for authenticated clients initialization

[DaevMithran]

Summary

Adds a Signer interface to @interledger/http-signature-utils and supports signer-backed authenticated clients in @interledger/open-payments. This allows KMS/HSM/non-extractable key integrations without changing the existing private-key API initialization

Changes

• Add Signer, PrivateKeySignOptions, and SignerSignOptions
• Support signing HTTP signature headers via external signer
• Add createAuthenticatedClient({ signer, keyId, walletAddressUrl })

Adds feature request raised in interledger/open-payments-go#47

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Latest commit: 2ecedea

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