Add protocol compliance tests for union encodings

[Baccata]

Some protocol compliance tests should be specified for the various encodings of unions that are specified in alloy :

https://github.com/disneystreaming/alloy/tree/main/modules/protocol-tests/resources/META-INF/smithy

[kubukoz]

for inspiration, here are some for open unions specifically:

https://github.com/disneystreaming/alloy/blob/f10713c06eb902938038a135401a5fc25e34b155/modules/protocol-tests/resources/META-INF/smithy/OpenUnions.smithy