[sai-gen] Update SAI API header file generation to use SAI spec

[r12f]

Moving SAI API header file generation from using the P4 table information to SAI spec.

This change contains several minor fixes to the SAI specs:

• Description and comment generation with respect to the SAI acronyms.
• Fixed SAI flags and default values.
• Added missing SAI attributes that is generated from table keys.
• Fixed valid only tag generation.
• Fixed unwanted header files for underlay.

Some typical changes (all comment changes, APIs are not changed and kept in the same order):

[r12f]

adding Ze into review, since his DPU image work is getting affected if the SAI API is not backward compatible.

[r12f]

@Pterosaur , without this change, the header generation will remain broken.