azure_exporter integration

pmm-managed, pmm part is contained in https://jira.percona.com/browse/PMM-7498 PRs.
pmm-agent, pmm-doc and pmm-api-tests part in open PRs here.

Suggested implementation:
1. Use modified azure exporter from https://jira.percona.com/browse/PMM-7678
2. Add new exporter type to pmm-agent
3. Modify pmm-admin add commands
4. Modify API
5. Add into pmm-managed.

It's required to fork an upstream exporter and made it compatible with PMM2.
Proposed community exporter:
https://github.com/RobustPerception/azure_metrics_exporter

The exporter requires a configuration file. We have to omit using the file and switch to work with internal PMM DB pmm-managed.