Implement ProvisioningRequest service

[yaroslava-serdiuk]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Implementing ProvisioningRequest service that is required for further implementation of ProvisioningRequest CRD proposal.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

https://github.com/kubernetes/autoscaler/blob/master/cluster-autoscaler/proposals/provisioning-request.md

[yaroslava-serdiuk]

/assign @kisieland

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In response to this:

/assign @kisieland

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[BigDarkClown]

Added some preliminary comments about code style and testing, but I don't think I have enough in-depth knowledge about Provisioning Requests to judge how this implementation satisfies the requirements. @kisieland, could you review this change as well?

[kisieland]

LGTM

[BigDarkClown]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BigDarkClown, kisieland, yaroslava-serdiuk

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• cluster-autoscaler/OWNERS [BigDarkClown]

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