Document libspdm requirements and assumptions for target compiler and system

[steven-bellock]

This is more to raise awareness rather than fix a bug, although we could fix the bug too since this is security-critical code. spdm_init_context zeros out spdm_context via zero_mem(spdm_context, sizeof(spdm_context_t)); Later on checks are performed such as if (spdm_context->local_context.local_cert_chain_provision[slot_id] == NULL) The C language does not guarantee that a pointer that has been memset to 0 evaluate to NULL. At the very least we should document this in a "language implementation decisions" document.

[steven-bellock]

Need a document in the doc/ folder that goes over assumptions.

[steven-bellock]

libspdm also assumes that the underlying encoding of character constants is ASCII.

[steven-bellock]

libspdm also assumes that the compiler supports pragma pack.

[steven-bellock]

libspdm assumes little endian architecture. Once that has been documented then this can be closed.