fix(connect-web-interceptor): connect v2 compatibility

[legau]

Summary

In Connect v2, messages are plain JSON objects so there are no method classes on them anymore. This PR adds JSON serialization in both ways.

Problem description

Using Connect v2 with grpc-web-devtools causes this error :

ConnectError: [unknown] req.message.toJson is not a function

Checklist

• Is this PR a reasonable size?

• For breaking changes: I have confirmed that other repos are not affected, or I have planned a deployment sequence to address any impact.

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