feat(events): add events extension

[phoban01]

What type of PR is this?

feature

Which issue does this PR fix:

#2992

What does this PR do / Why do we need it:
This PR introduces a new extension: events that enables publishing events in the CloudEvent schema.

The following event types are implemented:

• RepositoryCreated
• ImageUpdated
• ImageDeleted
• ImageLintFailed

Two event sinks are implemented as part of this change:

• HTTP: a generic http event sink that can be used for webhook style eventing
• NATS: a nats event sink that can send events to a nats server

Testing done on this change:

Unit tests added for http and nats event sinks. I have also tested things locally.

Automation added to e2e:

Will this break upgrades or downgrades?
No

Does this PR introduce any user-facing change?:

Yes it introduces a new configuration stanza for events.

The following shows how the extension may be configured:

  "extensions": {
    "events": {
      "enable": true,
      "sinks": [
        {
          "type": "nats",
          "address": "nats://127.0.0.1:4222",
          "timeout": "2s",
          "channel": "alerts.images"
       }, 
        {
          "type": "http",
          "address": "http://127.0.0.1:3333/events/zot/webhook",
          "timeout": "5s",
       }]
    }
  }

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[rchincha]

@phoban01 nice work! thanks for starting this PR.

[rchincha]

We really need to shorten the args list into a struct we can pass around. Not related to your PR thouugh.

[phoban01]

Updated with HTTP and NATs event sinks. Also generalized the sink config.

[rchincha]

what would credentials look like? and does it work?

[phoban01]

How the credential config is going to be used will be specific to the sink implementation.

But for example when connecting to a nats server I have verified that the following works:

  "extensions": {
    "events": {
      "enable": true,
      "sinks": [{
          "type": "nats",
          "address": "nats://127.0.0.1:4222",
          "timeout": "10s",
          "channel": "alerts",
          "credentials": {
            "username": "jane.joe",
            "password": "plain-text-or-encrypted-pass"
          }
      }]
    }
  }

If you don't want to pass credentials in the config then you can specify credentials.file and supply the credentials via the filesystem.

Alternatively you could use the TLS settings to configure a secure connection to the event system.

[rchincha]

^ this is a very good start.

[phoban01]

Anything I can do to help bump this along? cc: @rchincha