Unable to get az policy definition create --metadata to create an array value in policy definition

### Describe the bug

Not sure if this is a bug or feature request or docs gap. Will start with bug template.

When I create a policy definition using the portal or PowerShell with category `Guest Configuration`, the resulting policy definition correctly includes `"requiredProviders": ["Microsoft.GuestConfiguration"]` in the `metadata` section.

I am unable to get to this same state using `az policy definition create ... --metadata ....`.

I have tried a number of variations with different quoting and escaping schemes, including brackets in the value, not including brackets in the value, using a bash list, etc., but these all result in a policy definition where requiredProviders is some flavor of string, not an array. So far I have not found any examples online of someone getting `az policy definition create --metadata` to result in an array in the resulting policy definition.

Note that I am _not_ talking about passing multiple metadata key-value pairs to --metadata. That is easily done with space delimiting. I am talking about a single KVP where the value is an array. Is it possible? If so, can the working path please be documented (or --metadata updated to take JSON like --rules and --params, rather than using its own KVP representation scheme).

Thanks.

### Related command

`az policy definition create`

### Errors

There is no immediate error. Instead the command results in an undesired/invalid policy definition. For example, (depending on input scheme attempted) `"requiredProviders": "Microsoft.GuestConfiguration"` rather than `requiredProviders: [ "Microsoft.GuestConfiguration" ]`

### Issue script & Debug output

The output is extremely long due to my policy definition having a large set of params and rules logic. Not feasible to copy paste or get file to attach from isolated test environment. If possible, I will recreate the issue with a minimal policy definition and add that --debug output here.

### Expected behavior

`metadata` section of policy definition should have a key with an array value, such as `"requiredProviders": [ "Microsoft.GuestConfiguration" ]`

### Environment Summary

~ > az --version
azure-cli 2.61.0 *

core 2.61.0 *
telemetry 1.1.0

Extensions:
ai-examples 0.2.5
azure-iot 0.10.13
azure-sphere 1.0.0
ml 2.26.1
resource-graph 2.1.0
ssh 2.0.3

Dependencies:
msal 1.28.0
azure-mgmt-resource 23.1.1

Python location '/usr/bin/python3.9'
Extensions directory '/home/matthew/.azure/cliextensions'
Extensions system directory '/usr/lib/python3.9/site-packages/azure-cli-extensions'

Python (Linux) 3.9.19 (main, Mar 28 2024, 18:56:59)
[GCC 11.2.0]

Legal docs and information: aka.ms/AzureCliLegal

You have 2 update(s) available. They will be updated with the next build of Cloud Shell.

### Additional context

It looks like the underlying REST API takes actual JSON for the metadata input. Maybe the solution here would be to align `--metadata` in azcli with that (and with `--rules` and `--params` in cli, which also take actual JSON), rather than `--metadata` taking its own special space delimited format for KVPs.

Also, it seems weird that, input challenges aside, `az policy definition create` should be willing to create an invalid policy definition. I would have expected there is a schema somewhere which says `requiredParameters` is an array, not a string. Does that schema not exist? if it does exist, does `az policy definition create` not validate that the policy definition it creates is valid per the schema?

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