[202305] Parse subport value added to config_db (#3161)

[smagarwal-arista]

Description of PR

Cherry-pick https://github.com/sonic-net/sonic-swss/pull/3161 to 202305

PRs #18499 / #19314 Added support for subport values to be automatically calculated and inserted on creation of config_db.json. This change succeeds in adding the subport value into config_db.json - but it results in a log

WARNING #orchagent: message repeated 18564 times: [ :- parsePortConfig: Unknown field(subport): skipping ...]

being printed. This prevents the log being printed by handling the subport value.

Why I did it To prevent the orchagent log

How I verified it I manually tested on an Arista device that the log no longer appears