Implement leader election using client-go tools

[vicmarbev]

Signed-off-by: Víctor Martínez Bevià [email protected]

What this PR does / why we need it:

Implements leader election to improve availability

Which issue(s) this PR fixes:

Fixes #838

Does this PR introduce a user-facing change?

kapp-controller is now able to run multiple replicas thanks to leader-election mechanism

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.:

[vicmarbev]

I suppose changes in the CI would have to be made to run kapp-controller with more than one replica. I don't really get the workflow of running e2e locally with a local image. Can someone help me with this?

[vicmarbev]

I'm afraid I won't have much time in the following weeks to work on this, so I'm closing the PR so others can work on this issue. Thanks for the review, though!