Assign values to sai_switch_attr_t enum fields

This stack is towards supporting FDB Source User Meta.

This patch:

opencomputeproject#1259 introduced
explicit numbering to enum fields to avoid breaking warmboot across SAI
spec enhancements while maintaining readability. It fixed saiacl.h and
saihash.h.

Introduce similar explicit numbering for enum sai_switch_attr_t
Subsequent stacked commit needs to insert a new enum field in the
middle.

Signed-off-by: Shrikrishna Khare <skhare@fb.com>