betterproto: support `Struct` and `Value` (#551)

* betterproto: support `Struct` and `Value`

Signed-off-by: William Woodruff <william@trailofbits.com>

* betterproto: handle struct in to_dict as well

Signed-off-by: William Woodruff <william@trailofbits.com>

* tests: add Struct roundtrip tests

Signed-off-by: William Woodruff <william@trailofbits.com>

* specialize from_dict and to_dict on Struct

...rather than special-casing in the Message ABC.

Signed-off-by: William Woodruff <william@trailofbits.com>

* betterproto: `poe format`

Signed-off-by: William Woodruff <william@trailofbits.com>

* Update src/betterproto/__init__.py

Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>

* remove future annotations

Signed-off-by: William Woodruff <william@trailofbits.com>

* replace type[...] with typing.T

Signed-off-by: William Woodruff <william@trailofbits.com>

* quote instead

Signed-off-by: William Woodruff <william@trailofbits.com>

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Signed-off-by: William Woodruff <william@trailofbits.com>
Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>