[Extension] Add azmirror cloud endpoint for installing extensions in air gapped clouds and support config of index url

[fengzhou-msft]

Description

To support installing extensions in air-gapped clouds, we have set up a pipeline to sync the extension index, wheel packages and extension command tree to a public storage account called azmirror. A Teleport process will be set up to sync everything in azmirror to different storage accounts inside air-gapped clouds.

In order to use those files in each air-gapped cloud with Azure CLI, this PR adds an azmirror_storage_account_resource_id endpoint whose value may look like https://azmirroragc.blob.core.windows.net.
CLI will get extension related files from the endpoint if it is set in the current cloud, otherwise it will use the current default values (for public cloud and sovereign clouds).

This PR also adds config option extension.index_url to support customized extension index url. For example, to use the index.json on github:
az config set extension.index_url=https://raw.githubusercontent.com/Azure/azure-cli-extensions/master/src/index.json

Testing Guide

$ az cloud update --cloud-config "{\"endpoint_azmirror_storage_account_resource_id\":\"https://azureclireleases.blob.core.windows.net/releases\"}"
$ az cloud show  # you should see the value set for azmirrorStorageAccountResourceId
$ az extension list-available -otable --debug  # you should see a request sent to https://azmirror.blob.core.windows.net
$ az config set extension.index_url=https://raw.githubusercontent.com/Azure/azure-cli-extensions/master/src/index.json
$ az extension list-available -otable --debug  # Now you should see a request sent to raw.githubusercontent.com

History Notes

[Extension] Support config of extension index url

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[yonzhan]

Extension

[jiasli]

Maybe we can put these 2 URLs in one place?

[jiasli]

Also, isn't the whl location hard-coded in the index?

{
    "downloadUrl": "https://azurecliprod.blob.core.windows.net/cli-extensions/account-0.2.0-py2.py3-none-any.whl",
    "filename": "account-0.2.0-py2.py3-none-any.whl",
    "metadata": {
        "azext.isExperimental": true,

[fengzhou-msft]

The index will be teleported into different storage accounts in air gapped clouds, we do not have permission to modify the whl downloadUrl based on the destination storage accounts after it gets copied in air gapped clouds. So we need to modify the downloadUrl here in azure-cli when we need to download it in an air gapped cloud.

[jiasli]

Do we really need to put one URL in cloud config, one in az config?

[fengzhou-msft]

The one in cloud config is now a general support for downloading extensions and other tools/files needed in air gapped clouds. It's always a storage account endpoint and controlled by Microsoft.

The one in az config is to support customers that want to use their own customized index.json, which may have their own private extensions and some allowed public extensions. You can use any url of the index file, such as the raw content url on a github branch.

[yungezz]

put this to backlog now given environment setup is under process. When process done, will merge the PR.

[yungezz]

extension_pacakges_url?

[yungezz]

config overrides cloud setting