add leader election mechanism

[basit9958]

What this PR does / why we need it:

Added Leader Election Mechanism

OnStartedLeading: This callback is executed when the current instance becomes the leader. It starts running the controller logic by calling the Run function.
OnStoppedLeading: This callback is executed when the current instance is no longer the leader. It logs a message and may exit gracefully if needed.
OnNewLeader: This callback is executed when a new leader is elected. It logs a message indicating the new leader's identity.

Which issue(s) this PR fixes:

Fixes #838

Does this PR introduce a user-facing change?

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[neil-hickey]

Early Feedback: Can we move all the leader election code to a different file / module? Main ought to be as simple as possible (just start the app)

[basit9958]

okay will look into this

[neil-hickey]

Hi @basit9958 , thanks for this!

Is this PR intended to be for demonstration / Proof of Concept purposes - or the intended feature complete? If it's intended to be complete, looks like it doesn't compile right now.

+ go build -trimpath -mod=vendor -o controller ./cmd/controller/...
# github.com/vmware-tanzu/carvel-kapp-controller/cmd/controller
Error: cmd/controller/main.go:113:20: cannot use lock (variable of type *resourcelock.LeaseLock) as "context".Context value in argument to runLeaderElection: *resourcelock.LeaseLock does not implement "context".Context (missing method Deadline)
Error: cmd/controller/main.go:113:26: cannot use ctx (variable of type "context".Context) as *resourcelock.LeaseLock value in argument to runLeaderElection
Error: Process completed with exit code 1.

[basit9958]

Hi @basit9958 , thanks for this!

Is this PR intended to be for demonstration / Proof of Concept purposes - or the intended feature complete? If it's intended to be complete, looks like it doesn't compile right now.

+ go build -trimpath -mod=vendor -o controller ./cmd/controller/...
# github.com/vmware-tanzu/carvel-kapp-controller/cmd/controller
Error: cmd/controller/main.go:113:20: cannot use lock (variable of type *resourcelock.LeaseLock) as "context".Context value in argument to runLeaderElection: *resourcelock.LeaseLock does not implement "context".Context (missing method Deadline)
Error: cmd/controller/main.go:113:26: cannot use ctx (variable of type "context".Context) as *resourcelock.LeaseLock value in argument to runLeaderElection
Error: Process completed with exit code 1.

It is meant to be complete, I will fix these errors