Yaml analyzer task

We often have updates that require changes in YAML...

Examples:
- Deprecated agent pools
- schema changes (phases -> jobs)
- etc...

… or new features that would require someone to opt-in...

Examples:
- enableTelemetry
- enablePublishBuildArtifacts
- publishBuildAssets (to BAR)

… or some YAML value that is missing or incorrect...

Examples:
- helixSource
- missing a clean step

If we had a custom YAML analyzer task that we could provide via templates, or a team could directly add to their yaml, then we could provide an immediate (and persistent) way for teams to be notified of upcoming changes or ways that their builds deviate from standard.

ie, we could analyze yaml, determine that agent pool "X" is being deprecated, and provide a build warning with how to fix the warning, a date when "X" is going away, and where to get more information. A nice affect of this is that, if you have a build that doesn't run very often, and then a month later, you run it, the build will tell you what needs to change.