msgbuf: Define a separate msgbuf structure for encode/decode function

Define separate msgbuf structures to avoid casting away const-qualifiers
in the msgbuf constructor function:

* pldm_msgbuf_rw:

msgbuf: Define a separate msgbuf structure for encode/decode function

Define separate msgbuf structures to avoid casting away const-qualifiers
in the msgbuf constructor function:

* pldm_msgbuf_rw: for encode functions with non const-qualified buffer
* pldm_msgbuf_ro: for decode functions with const-qualified buffer

Further, use _Generic() to keep the API ergonomic while still yielding a
compile error when wrong msgbuf type is passed.

Change-Id: I71dbcb7996e9fb402b49870fce539a939c1497e5
Signed-off-by: John Chung <john.chung@arm.com>
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>

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