Cleans up command options package

[JoshVanL]

Managing options for the kube-oidc-proxy has become quite unwieldy which makes it difficult to work on and add new flags. This PR updates the options package to return a single options struct which provides a much better interface for the consuming package. This PR also moves more into the options package where it makes sense which cleans things up a bit more.

/assign @simonswine

[jetstack-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JoshVanL

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[simonswine]

Needs rebase

/assign @JoshVanL /unassign

[JoshVanL]

/unassign /assign @simonswine

[JoshVanL]

Think I addressed your comments /unassign /assign @simonswine

[JoshVanL]

/retest

[simonswine]

Nice on, thanks

/lgtm