Added worker ID field to build

[applejag]

• I've added a new note in the CHANGELOG.md file, according to docs:
  https://iver-wharf.github.io/#/development/changelogs/writing-changelogs

Summary

• Added worker ID to build
• Added config.CI.Engine.API enum to declare what kind of engine API it is.
• Parse worker ID from build trigger response if engine is wharf-cmd.v1

Motivation

Adds worker ID support.

wharf-cmd-watchdog needs this value, and we need this value later in #154 when cancelling builds.

Based on RFC-0025: wharf-cmd provisioning

[applejag]

Wow the CodeQL scan noticed through 20 different nested function calls that we used c.GetQuery("stage") all the way down to when we construct the trigger URL where we do fmt.Sprintf("%s&%s=%s", ...)

https://github.com/iver-wharf/wharf-api/pull/156/checks?check_run_id=5361232036

However it failed to notice that we did pass the value through url.QueryEscape, so I marked it as "false positive"

[Alexamakans]

Good stuff 👍🏻