support passing []byte as TPM2B

Capturing one more bit of feedback @matt-tsai gave us in his internship report -

Occasionally, someone sending a TPM2B to a TPM would benefit from not having had to deserialize or serialize it (e.g., the 2B came from a protobuf and can just be forwarded to the TPM command). Every TPM2B_X should support being instantiated either with its contents or a flat buffer. Go1.18 generics can help maintain type safety and compiler-time checking here.