feat: (WIP) Application dependencies (#3517)

[jannfis]

PoC for #3517 - up for discussion. Things to note:

• Currently, manual syncing is also blocked when there are unmet dependencies
• Not a single test yet

Credit where credit is due: Dependency resolution was taken from http://dnaeon.github.io/dependency-graph-resolution-algorithm-in-go/

Enables to specify dependencies between Applications:

dependsOn:
  - applicationName: otherApp
  - applicationName: someApp

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Optional. My organization is added to USERS.md.
• I've signed the CLA and my build is green (troubleshooting builds).

[codecov]

Codecov Report

Merging #3892 into master will decrease coverage by 0.16%.
The diff coverage is 3.94%.

@@            Coverage Diff             @@
##           master    #3892      +/-   ##
==========================================
- Coverage   42.44%   42.27%   -0.17%     
==========================================
  Files         120      120              
  Lines       17567    17642      +75     
==========================================
+ Hits         7456     7459       +3     
- Misses       9161     9229      +68     
- Partials      950      954       +4     

Impacted Files	Coverage Δ
pkg/apis/application/v1alpha1/types.go	60.00% <0.00%> (-0.14%)	⬇️
controller/sync.go	27.60% <3.50%> (-12.96%)	⬇️
controller/appcontroller.go	43.21% <5.88%> (-0.73%)	⬇️

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