implement tenants endpoint for victorialogs

[dmitryk-dk]

Describe Your Changes

Implemented endpoint like VictoriaMetrics has /select/admin/tenants. This endpoint returns tenants from the storage as slice of strings like []{"0:0", "1:0"}.
This endpoint can help to build UI with requests per tenant.

{
  "status": "success",
  "data": [
    "0:0",
    "2:1",
    "3:1"
  ]
}

Checked both solutions, single version and cluster. Fixed all comments from this PR

Checklist

The following checks are mandatory:

• My change adheres to VictoriaMetrics contributing guidelines.
• My change adheres to VictoriaMetrics development goals.