Update design and expose new attributes for the CustomTypeMarshaller support

[jkoritzinsky]

Update the design of the V2 marshallers based on API review feedback.

Expose the CustomTypeMarshallerAttribute class and the MarshalMode enum so we can use them to update ASP.NET Core's custom marshallers and delete the V1 model sooner.

Also block the analyzer from warning on the V2 design for now until we actually get it updated.

Contributes to #70859

[ghost]

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Issue Details

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Update the design of the V2 marshallers based on API review feedback.

Expose the CustomTypeMarshallerAttribute class and the MarshalMode enum so we can use them to update ASP.NET Core's custom marshallers and delete the V1 model sooner.

Contributes to #70859

Author:	jkoritzinsky
Assignees:	-
Labels:	area-System.Runtime.InteropServices, source-generator
Milestone:	-

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[jkoritzinsky]

Failures are a network blip and #71684