Add snippet updater

[xiangyan99]

No description provided.

[scbedd]

I'd really appreciate a little code comment or something that maps out what is actually happening here. I am not convinced of this code as-is just from reading it.

A snippet is the complete code from the actual python file. So ripped directly from the python file it'll look like.

        # [START trio]
        from azure.core.pipeline.transport import TrioRequestsTransport

        async with AsyncPipeline(TrioRequestsTransport(), policies=policies) as pipeline:
            return await pipeline.run(request)
        # [END trio]

And we need to remove spaces, as we will be emplacing the snippet into the markdown file.

First question, why do we start with the second character of the snippet? Shouldn't we start with the first?

        if snippet[-1] == "\n":
            snippet = snippet[:-1]

I seriously do not get it.

[scbedd]

I'm too dumb to track what the code is actually doing with the indexes, needs some explanation. The test additions etc are great and I'll approve once I understand the replacement.

[xiangyan99]

Close the PR and use Azure/azure-sdk-for-python#26899 to track the code change.