Chart: Allow users to supply their own webhook secret

[mkantzer]

Description

Creates a new value supplied for agentInjector.certificate.method, that fully disables the generation of a secret. This prevents an apply error when helm would try to create an already-existing secret with annotations indicating ownership by a system other than this chart.

Checklist

• [x] I made sure to update ./CHANGELOG.yml.
• [x] I made sure to add any docs changes required for my change (including release notes).
• [ ] My change is adequately tested.
• [x] I updated CONTRIBUTING.md with any special dev tricks I had to use to work on this code efficiently.
• [x] I updated TELEMETRY.md if I added, changed, or removed a metric name.
• [ ] Once my PR is ready to have integration tests ran, I posted the PR in #telepresence-dev in the datawire-oss slack so that the "ok to test" label can be applied.

I'm not sure on the process of testing helm charts here, so I'd appreciate some help there.

[mkantzer]

@thallgren I don't have write access, so I can't merge. Is there anything else I need to do, or will you pull this in when ready?