Support for enum flags

[andrueastman]

Taking a look at the metadata for the SourceType enum in the graph metadata, there exists the IsFlags property which is set to true.

      <EnumType Name="sourceType" IsFlags="true">
        <Member Name="mailbox" Value="1" />
        <Member Name="site" Value="2" />
      </EnumType>

The flags attribute is not propagated to the converted openApi description in any way at the moment and also information on the values is also lost in the conversion process.

    microsoft.graph.security.sourceType:
      title: sourceType
      enum:
        - mailbox
        - site
        - unknownFutureValue
      type: string

This poses a challenge to Kiota at the moment as there is no signal for the generator to generate an appropriate Flag property.

Notes

• According to Separate annotation per enum value OAI/OpenAPI-Specification#348 (comment) enums could be better described to cater for values by conforming to the json schema
• Some extensions like x-enumFlags to denote flagged enums
• AutoRest uses [x-ms-enum](https://github.com/Azure/autorest/tree/main/docs/extensions#x-ms-enum) to denote values and fields.

Related to microsoftgraph/msgraph-beta-sdk-dotnet#496

cc @baywet @darrelmiller

[baywet]

We're using the x-ms-enum extension when Aliasing the code symbol and the serialized value.
It'd be great if we could pr their specification to add the information there instead of coming up with our own.
Also we should check whether oai 3.1 or Json schema has something to support that notion which arguably is very dotnetty.

[darrelmiller]

I think doing a PR to the x-ms-enum would be the right thing. If we add "isFlagEnum" and set the modelAsString to true then that would cover all cases.

@andrueastman Could you create a PR for this change? You could reach out to Mike Kistler to help you get eyes on the issue.

[moh-hassan]

Is there any Update to this issue.
Adding x-ms-enum extension is enough as described in AutoRest.
I set the property AddEnumDescriptionExtension=true in the setting , but it has no effect.