Implement internal API for KAS to check if agent has project access

What does this MR do and why?

Implement internal API for KAS to check if agent has project access

This is the rails part of Implement internal GitLab API to verify agent p... (gitlab-org/cluster-integration/gitlab-agent#401 - closed)

The KAS part is in Use new Rails API to verify agent project access (gitlab-org/cluster-integration/gitlab-agent!968 - merged)

Refs: gitlab-org/cluster-integration/gitlab-agent#401 (closed)

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