Expanding UnMarshall + a few additions

[ionut-arm]

Hi all,

This PR contains two commits:

• first one adds another method to the UnMarshall trait to allow it to unmarshall from the buffer at a given offset, and return the post-unmarshalling offset back; it also adds Marshall/UnMarshall for CommandCode and u32
• second one adds CommandTag as another StructureTag, as well as Marshall/UnMarshall for this new enum, and for the AttestationType enum

Obviously, also adding tests for the new stuff. The plan is to modify the remaining UnMarshall implementations to use unmarshall_offset as the "base" implementation, and unmarshall to just use that underneath.

Let me know if you'd like me to split out the 2nd commit - I've put them together in one PR since the 2nd depends on the 1st.

[ionut-arm]

I've had to update the _offset versions of the methods because they were hardcoded to u64, whereas the original size_t type is 32 bits on some platforms.

[ionut-arm]

Also it seems the log crate has been bumped beyond our MSRV underneath our feet, so we have to decide whether we want to bump the MSRV or fix the log version.

cc @gowthamsk-arm

[Superhepper]

Also it seems the log crate has been bumped beyond our MSRV underneath our feet, so we have to decide whether we want to bump the MSRV or fix the log version.

cc @gowthamsk-arm

It would be nice if we can lock it to a specific version but I am also completely ok with bumping MSRV.