Azure repository: Move to named configurations as we do for S3 repository

[dadoonet]

We should have the same behavior for Azure repositories as we have for S3 (see #22762).

Instead of:

cloud:
    azure:
        storage:
            my_account1:
                account: your_azure_storage_account1
                key: your_azure_storage_key1
                default: true
            my_account2:
                account: your_azure_storage_account2
                key: your_azure_storage_key2

Support something like:

azure.client:
            default:
                account: your_azure_storage_account1
                key: your_azure_storage_key1
            my_account2:
                account: your_azure_storage_account2
                key: your_azure_storage_key2

Then instead of:

PUT _snapshot/my_backup3
{
    "type": "azure",
    "settings": {
        "account": "my_account2"
    }
}

Use:

PUT _snapshot/my_backup3
{
    "type": "azure",
    "settings": {
        "config": "my_account2"
    }
}

If someone uses:

PUT _snapshot/my_backup3
{
    "type": "azure"
}

It will use the default azure repository settings.

And mark as deprecated old settings.