gitops-engine feat: Add sync-dependencies

This PR introduces a new feature: sync dependencies.

Sync dependencies give a new way to order sync operations that is influenced by sync waves and sync hooks (all of which I wrote, so I know it extremely well).

Like sync waves, a sync operation will only progress when all the dependents have completed.

Dependencies between objects is specified by the new argocd.argoproj.io/sync-dependencies annotation.

# The dependencies are specified as a comma-separated list of objects.
# The format of each object is <group>/<kind>/<namespace>/<name>
# The group and kind are optional. If not specified, they'll match any group or kind.
# The namespace is optional. If not specified, the namespace is assumed to be the same as the namespace of the object.
# The name is required.
argocd.argoproj.io/sync-dependencies: a,b,c

Reasons to do this:

The 3rd most popular enhancement request for Argo CD
Explanation can be found in argo-cd#3517.
Implementation is contained within the GitOps engine.

Reasons not to do this:

Impacts core GitOps engine code (must be mitigated by exceptional testing).
Another gun to shoot yourself in the foot with.

Open questions:

Sync waves, hooks, and dependencies have a certain level of opacity to them. By which I mean, that it is not always clear what they're doing unless you're familiar with the app. I wonder if we should introduce a new "waiting" state for resources?
Should dependencies take precedence over waves? I think this is yes.
How should dependencies be described? I've used annotation, is that right? Is that the right structure?
Are the rules that match tasks to dependencies?

Mar 19 '23 23:03 alexec

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (ed70eac) 55.75% compared to head (745fb5a) 55.61%. Report is 18 commits behind head on master.

:exclamation: Current head 745fb5a differs from pull request most recent head de3f8c9. Consider uploading reports for the commit de3f8c9 to get more accurate results

Files	Patch %	Lines
pkg/sync/sync_context.go	64.28%	4 Missing and 1 partial :warning:
pkg/sync/sync_tasks.go	94.73%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
- Coverage   55.75%   55.61%   -0.15%     
==========================================
  Files          41       42       +1     
  Lines        4525     4531       +6     
==========================================
- Hits         2523     2520       -3     
- Misses       1808     1817       +9     
  Partials      194      194

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