[receiver/azuremonitorreceiver] feat: multi subscriptions support and automatic discovery

[celian-garcia]

Recreated from a diffferent fork. Context: #36467

Description

This PR allows to discover and scrape all the subscriptions located in the tenant.
I'm adding also at the same time the ability to give multiple subscriptions as it's free and can also be desired.

This is a part of this PR: #29593 that has been split for readability.
Indeed in a next PR, I'll propose the usage of the getBatch of metrics Azure API. Because if your tenant contains a lot of subscription, you can face rate limitations.

It contains also a little refactor of the tests and the "backdoor" used to mock the API. Now the tests are using the way provided by Azure: the fake API. https://github.com/Azure/azure-sdk-for-go/tree/main/sdk/samples/fakes
The "backdoor" is now only the client options. So no weird interfaces nor weird constructor for the Azure clients inside the scraper.go file.

Link to tracking issue

Fixes #36612

Testing

Documentation