✨ Implement Priority and Serial settings for discovered handlers

[Danil-Grigorev]

What this PR does / why we need it:

In-place upgrades defines machine-updates process as looping until completion for each registered updater.

In order to leverage runtime extensions in processing these updaters, handler execution needs to be able to block (retry until completion) on individual updater, in case it responds with pending status before requesting other unfinished hooks. This PR introduces serial setting for the handler discovery.

Additionally, current handlers list is not guarantied to be ordered, so it may cause a situation when:

• Updater A has finished
• Updater B is serial and is pending
• Updater C has not started (waiting on B)

On the second iteration:

• Updater A has finished
• Updater C has finished (jumped the queue and skipped waiting on B)
• Updater B is serial and is pending

To address this, a priority field is added, which allows updaters to declare execution order.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes rancher/highlander#118

/area runtime-sdk

[k8s-ci-robot]

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[k8s-ci-robot]

PR needs rebase.

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