`prowjob-dispatcher`: refactor dispatcher and create a client for use by other tools

[smg247]

This moves the server and necessary logic into the pkg/dispatcher, and adds a client. This is necessary for tools like payload-testing's prpqr_reconciller and multi-pr-prow-plugin to be able to know which cluster to set on job's they create.

/cc @jmguzik @openshift/test-platform

[smg247]

/label tide/merge-method-squash

[openshift-ci-robot]

/test remaining-required

[smg247]

/retest

[openshift-ci[bot]]

@smg247: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/security	ea5a12c0aab79d89071c23aca6c44f49cb307395	link	false	/test security

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[Prucek]

/lgtm /hold for @jmguzik to review but if you are blocked with this, feel free to unhold, it's nicely done

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jmguzik, Prucek, smg247

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[smg247]

/hold cancel