[Extension] Support automatically installing an extension if the extension of a command is not installed

[fengzhou-msft]

Description

When a command failed in parser._check_value due to a command word is not in the choice list of its previous word, such as az vm abc would fail because abc is not a choice under vm. This failure could be caused by:

1. Typo, e.g. az accuont show or use get instead of the correct show.
2. The command is from an extension but the extension is not installed.

Previously, we cannot tell the difference and the error message is like this:

az: 'bluprint' is not in the 'az' command group. See 'az --help'. If the command is from an extension, please make sure the corresponding extension is installed. To learn more about extensions, please visit https://docs.microsoft.com/en-us/cli/azure/azure-cli-extensions-overview

This PR adds a search for the unmatched command in all the extension commands if a close match for the wrong command is not found in all module commands (indicating it's not likely a typo). If the command (not including options) matches a complete extension command, based on config, we can automatically install the extension with or without prompt. Then the command will continue to run by default or users can choose to rerun by themselves.

A mapping of all extension commands to extension names will be fetched from a storage account and stored in ~/.azure/extCmdIndex.json. It is valid for 10 days and will only be refreshed when a parse error happens. An example of the file:

{
       "aks": {
                "create": "aks-preview",
                "update": "aks-preview",
                "app": {
                    "up": "deploy-to-azure"
                },
                "use-dev-spaces": "dev-spaces"
        },
        ...
}

Configuration:
Both config options are under extension section:

1. use_dynamic_install
   yes_prompt: it will use dynamic extension install and prompt to user whether they would like to install the needed extension.
   yes_without_prompt: it will install the needed extension directly without confirmation from users.
   no: it falls back to previous behavior and just throws an error. This is the default for now.

2. run_after_dynamic_install
   yes/true: It’s the default and will continue to run the command after extension is installed.
   no/false: It will stop when extension is installed and ask the user to rerun the command.

You can also set with environment variables (AZURE_EXTENSION_USE_DYNAMIC_INSTALL, AZURE_EXTENSION_RUN_AFTER_DYNAMIC_INSTALL)and they have higher priority.

Testing Guide

1.set env var to turn on the feature:
export AZURE_EXTENSION_USE_DYNAMIC_INSTALL=yes_promt
2.run a command from an extension not installed to chekc the behavior.
3.run a real error command to chekc the new clear error message.

---

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[yonzhan]

Extension

[arrownj]

Is it possible to add test for this change ?

[arrownj]

what if ext_name is None ?

[fengzhou-msft]

If ext_name is None, error_msg will be None and below code will use the {prog}: '{value}' is not in the '{prog}' command group. for error message.

[fengzhou-msft]

Is it possible to add test for this change ?

Test added.

[arrownj]

LGTM

[haroldrandom]

cli_ctx seems could be None and then line 342 would crash.

[fengzhou-msft]

line 342 has the logic if cli_ctx else 'no' to handle None, it will use 'no' as the value.

[haroldrandom]

Would log at warning level be better?

[fengzhou-msft]

For now, let's just fall back to previous behavior when something is wrong as the warning may confuse users when they only typed a wrong command.

[haroldrandom]

Would log at warning level be better?