Add api

[ltitanb]

Adds a formal Signer API spec and a simple client implementation. To clarify the naming:

• the Signer module implements the Signer API and provides a light abstraction over different signing methods
• Commit Modules call the Signer API to request signatures , etc

Currently the default SignerClient can be used, it expects a url endpoint and a jwt for auth, which will need to be passed by the launcher eg via ENV variables or config

[David-Petrov]

Seems to me that the signer-api.yml and the new Signer client implementation regard a new version of the API:
cb_common::commit::client::GET_PUBKEYS_PATH = "/signer/v1/pubkeys"
cb_common::commit::client::REQUEST_SIGNATURE_PATH = "/signer/v1/request_signature"
whilst the actual HTTP service (the signer module) under cb_crypto::service::SignerService implements the old API:
cb_common::pbs::constants::PUBKEYS_PATH = "/signer/pubkeys"
cb_common::pbs::constants::SIGN_REQUEST_PATH = "/signer/sign"

Those two pairs of paths should be the same ones, shouldn't they? Or am I missing something?

Also, is the da_commit example module intentionally left unmodified?

Apart from these, everything looks good! 👍

[David-Petrov]

LGTM 👍