Add savepoint API to DbTransaction

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Closes #33397

Note that I've continued the general ADO.NET pattern of having the default implementation of async methods fall back to the sync methods. This also means that the default NotSupportedException gets returned as a failed task rather than a synchronous thrown exception.

[Dotnet-GitSync-Bot]

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.

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Based on the bot's gentle reminder, added triple-slash comments with documentation for the new APIs. Last time I did this, I had to submit a separate PR to https://github.com/dotnet/dotnet-api-docs - has some automatic sync been put into place? If not it may be better not to have the docs here and worry about keeping them in sync...

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Rebased and made the proper modifications after approval of the design proposal (#33397).

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@ajcvickers ping