[ART-1618] [ART-2800] New signing mechanism

[vfreex]

This PR adds a new signing mechanism to let promote-assembly job directly send signing requests through UMB.

This eliminates the intermediate step that pulling artifacts from the internet.

Basically this PR gives pyartcd the ability to sign things with pure Python implementation. So that we will not need a dedicated sign-artifiacts job and also eliminate the need to hand over artifacts between jobs. Concretely, it
comes with a class AsyncUMBClient aiming to simplify send and receive UMB messages using request/response model. (Existing python library is hard to use).

uri = "stomp+ssl://umb.stage.api.redhat.com:61612"
cert_file = "ssl/nonprod-openshift-art-bot.crt"
key_file = "ssl/nonprod-openshift-art-bot.key"  
async with AsyncUMBClient(uri, cert_file, key_file) as umb_client:
    # send a message
    await umb_client.send(topic, request_body)
    # receive messages
    receiver = await umb_client.subscribe(consumer_queue, subscription_name)
    async for message in receiver.iter_messages():
        print(message.headers["message-id"])

class AsyncSignatory sits on top of class AsyncUMBClient , which can be called from a python script to sign artifacts:

uri = "stomp+ssl://umb.stage.api.redhat.com:61612"
cert_file = "ssl/nonprod-openshift-art-bot.crt"
key_file = "ssl/nonprod-openshift-art-bot.key"
async with AsyncSignatory(uri, cert_file, key_file, sig_keyname="beta2", requestor="yuxzhu") as signatory:
    pullspec = "quay.io/openshift-release-dev/ocp-release:4.11.31-x86_64"
    digest = "sha256:cc10900ad98b44ba432bc0d99e7d4fffb5498fd6844fc3b6a0a3552ee6d64059"
    # sign a release payload
    with open("signature-1", "wb") as sig_file:
        await signatory.sign_json_digest("openshift", "4.11.31", pullspec, digest, sig_file)
    # sign a message digest
    with open("sha256sum.txt", "rb") as in_file, open("sha256sum.txt.gpg", "wb") as sig_file:
        await signatory.sign_message_digest("openshift", "4.11.31", in_file, sig_file)

promote-assembly is updated to use AsyncSignatory. sign-artifacts job is not used but I haven't removed it.

[vfreex]

This change is not related but let me explain why it is here.

When I attempted to store UMB brokers configuration in artcd.toml, I met some bugs in
toml library. It can't correctly parse configs with nested dicts and arrays.

Considering toml is not actively maintained and tomli has been adopted as a standard library in Python 3.11, I think we should move to tomli.

At the end, I decided to write a URI parser to configure UMB brokers using syntax like stomp+ssl://example.com:12345, so this became an irrelevant change.

[locriandev]

Same as above, I would rather move the dry_run check right at the beginning of the function

[vfreex]

Committed some minor fixes and tested with 4.11.46 promotion: https://saml.buildvm.hosts.prod.psi.bos.redhat.com:8888/job/hack/job/yuxzhu/job/aos-cd-builds/job/build%252Fpromote-assembly/106/consoleFull

[locriandev]

LGTM

[vfreex]

Thanks for the review.
Rebased. Merging...