[SAI] Add support for dash configuration from sai interface

[kcudnik]

Adding this support will allow syncd to easy pass configuration parameters to dash libsai using service method table

[chrispsommers]

Code LGTM but I am not very familiar with the usage of these APIs. Can you give an example how they might be useful for DASH?

[kcudnik]

for dash it should be transparent, main use case is for syncd, for example we load profile.ini file which configure many different aspects of libsai, in this case i allow to configure some dash api, like every variable in config.h and saidash.h keys, you can take a look of use case in syncd here: https://github.com/sonic-net/sonic-sairedis/blob/master/syncd/Syncd.cpp#L3165 when we load profile from file, if this config will be supproted in dash then we can modify easy those parameters from syncd pointing to right configuration files

most important line is here: https://github.com/sonic-net/sonic-sairedis/blob/master/syncd/Syncd.cpp#L3218

btw. all those SAI/template files i will move to proper classes, but it will take some time, and i dont want to do it in one big PR, i will take steps to it, so maybe at this point it could make confusion, but in my head and couple PRs in the future this works, trust me, im Jack Sparrow XD

[kcudnik]

can we have this merged ?

[kcudnik]

thanks!