feat: support for external secrets

[DrackThor]

What:

• enable support for providing broker token, snyk token and scm token via external secret

Why:

• I don't want to have to put my secret values in a values.yaml stored in git when deploying via gitops

[CLAassistant]

All committers have signed the CLA.

[soniqua]

Hi @DrackThor - thanks for your contribution to the Snyk Broker Helm repo.

We've been working internally on a similar solution, and I've had time to push that up today (#134). I think that should satisfy the need you outline here, and maintain backwards compatibility with how names were generated with previous versions of the chart.

[DrackThor]

@soniqua great to hear that! In that case I'll close this PR since the feature was added meanwhile.