feat(auto-cancel): add pipeline and yaml field for auto cancel

[ecrupper]

This change allows users to specify an auto_cancel policy in their pipeline metadata. This policy will impact whether or not running and/or pending builds that may be stale/obsolete will be canceled.

For example, let's consider a pipeline like this

version: "1"

metadata:
    auto_cancel:
        pending: true
        running: true
        default_branch: false

steps:
    - name: example
       image: alpine:latest
       commands:
           - echo "hello"

If I were to push twice to a feature branch in quick succession using this pipeline, the first push build would be canceled in favor of the second.

pending: obsolete builds in a pending state will be canceled (default to true unless the entire block is not provided, then default to false).
running: obsolete builds in a running state will be canceled
default_branch: obsolete push builds to the default branch will be canceled

Draft server PR: go-vela/server#911

[codecov]

Codecov Report

Merging #299 (3224f21) into main (9b2ca77) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #299   +/-   ##
=======================================
  Coverage   97.08%   97.08%           
=======================================
  Files          62       62           
  Lines        6816     6837   +21     
=======================================
+ Hits         6617     6638   +21     
  Misses        147      147           
  Partials       52       52           

Files	Coverage Δ
yaml/metadata.go	100.00% <100.00%> (ø)