[roll] Roll fuchsia [fidl][python] Add support for framework errors and empty success structs

Some FIDL protocol methods don't return a value. In Python, the usual
way to represent this is to return None, but this is complicated by the
FIDL IR providing empty success structs for those cases.

To ensure these empty structs are not present in bindings, this
implementations runs an initial pass through all structs in the IR to
compile all empty success structs into the type None. After this pass,
subsequent compilation will skip empty success structs.

This change also introduces complication of ExternalStructs from
fidlgen.Root. This is because ExternalStructs is known to contain empty
success structs from fuchsia.unknown.

Test: Add a surface level test suite for the response and result types
for protocol methods. A later change will actually implement protocol
methods.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1231244
Original-Revision: fa2bbf43a8ddc98cf0ef3464883e7375a1e3cf2a
GitOrigin-RevId: e3ba946174c2f7b84b7d8c16a5ee305bc6bf8e28
Change-Id: I997ab69278a38302e6a3c73a2eb1920e764106a6