[0.12.x] Publicly expose TypedTransactions

[kclowes]

What was wrong?

This is the v0.12.x back port of publicly exposing TypedTransactions. This should be backwards compatible to allow a nice upgrade path.

How was it fixed?

I cherry-picked the commit that moved typed transactions out of the _utils folder, and then added a DeprecationWarning in eth_account/_utils/typed_transactions/__init__.py that is triggered when typed transactions get imported via _utils. This felt like extra insurance against the breaking changes coming in 0.13.

Todo:

• [x] Clean up commit history
• [x] Add or update documentation related to these changes
• [x] Add entry to the release notes

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