Add support for Vault Agent's Auto-Auth feature

[jmcconnell]

Vault Agent provides an Auto-Auth capability where the agent is responsible for managing and using the token but a proxy API is provided so that clients no longer need to submit an X-Vault-Token header with requests. quarkus-vault is currently configured to omit this header if the configured token is null, but I don't see a safe way of eliciting this behavior (please correct me if I've overlooked something.)

Would it be possible to update the VaultRuntimeConfig#authentication() signature to return an Optional<VaultAuthenticationConfig> and add a new configuration property that allows this to be None? My company's internal cloud service has stopped passing wrapped client tokens to cloud services in favor of an Auto-Auth-enabled Vault Agent sidecar, so I don't currently see another authentication option that will work with quarkus-vault, as-is.