feat: implement server-side signing via hashicorp vault service (#41)
* feat: integrate HashiCorp Vault for mnemonic key management * feat: enhance VaultAdapter to support generic secret types * feat: update mnemonic generation to accept passphrase and refactor Secret struct * feat: introduce signing module with HashiCorp Vault integration for secret management * refactor: get key via config and remove unnecessary enums * refactor: reorganize module imports and improve formatting in main.rs * feat: update SecretAdapter to accept a key parameter and enhance Config with server_signing option * refactor: update transaction signing to use mnemonic directly and clean up imports * refactor: remove direct instantiation of HashiCorpVaultClient in main and move to pipeline run function * refactor: replace SecretAdapter with SigningAdapter and streamline transaction signing process * refactor: update SigningAdapter to accept owned data and improve transaction signing logic * refactor: replace KeyValuePairs with BTreeMap type * chore: update pallas dependency feature flag * refactor: update phase one and phase two validation functions to use new feature flag names