feat: sync private repository secrets to managed agents

[chetan-rns]

What does this PR do / why we need it:

• Add informers to both agent and principal to reconcile repository secrets
• Propagate the repository secrets to the correct target agent based on AppProjects.
• Cache the mapping between repos to agents in case the AppProject is deleted.
• Add unit tests and e2e tests

Which issue(s) this PR fixes:

Fixes #474

How to test changes / Special notes to the reviewer:

1. Create an AppProject on the principal cluster targeting one of the managed agents.
2. Create a repository secret referring to the newly created AppProject
3. Repository must be synced to the managed agent.

Checklist

• Documentation update is required by this PR (and has been updated) OR no documentation update is required.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 42.69311% with 549 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.23%. Comparing base (398a271) to head (2ec11ef).

Files with missing lines	Patch %	Lines
internal/manager/repository/repository.go	18.97%	109 Missing and 2 partials ⚠️
internal/resync/resync.go	11.82%	79 Missing and 3 partials ⚠️
internal/manager/manager.go	0.00%	75 Missing ⚠️
...ternal/backend/kubernetes/repository/kubernetes.go	34.44%	59 Missing ⚠️
agent/inbound.go	62.67%	39 Missing and 14 partials ⚠️
principal/server.go	38.55%	42 Missing and 9 partials ⚠️
principal/callbacks.go	74.41%	38 Missing and 6 partials ⚠️
agent/agent.go	16.66%	23 Missing and 2 partials ⚠️
internal/event/event.go	0.00%	17 Missing ⚠️
agent/outbound.go	83.09%	11 Missing and 1 partial ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #526      +/-   ##
==========================================
- Coverage   45.16%   44.23%   -0.93%     
==========================================
  Files          88       89       +1     
  Lines       10945    11757     +812     
==========================================
+ Hits         4943     5201     +258     
- Misses       5611     6129     +518     
- Partials      391      427      +36     

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[jannfis]

I'm just testing this PR locally, and most of the things work really great.

I noticed one thing:

• I have scoped a repository to a project. The project itself allows the agent-managed cluster.
• The repository gets propagated to the agent-managed agent, and is created on the target cluster.
• I stop the principal temporarily and make a change to the project, removing the agent-managed from the list of allowed destinations.
• I start the principal again. The agent reconnects, but the repository credentials are never removed from the agent-managed cluster

[chetan-rns]

I start the principal again. The agent reconnects, but the repository credentials are never removed from the agent-managed cluster

@jannfis I've fixed the resync issue. Also added a few e2e tests to verify the resyncing of AppProject/Repository on restarts.

[jannfis]

LGTM.

Awesome PR, @chetan-rns 🙇