Pass VM config through API instead of filesystem

[taraskozub-percona]

Description

What should be done: pmm-managed should pass Victoria Metrics config using API instead of filesystem

Benefit: it will remove one more dependency on the file system

Suggested solution

Suggested implementation: -promscrape.config flag of Victoria metrics supports HTTP endpoints, so we should reconfigure it to use API for it, we should create a new endpoint in pmm-managed and point victoria metrics to that endpoint.

Additional context

No response

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