LG-6925: Move client_id field for USPS API from hardcoded value to config

[NavaTim]

This pull request sources the client_id value from the config instead of hardcoding it. See LG-6925 for more details.

I plan to have the config deployed with the client_id to each environment before or during the deployment of this code (and will coordinate w/ others before merging to make sure that happens).

[aduth]

LGTM 👍

[svalexander]

I was able to find a bit more on where client_id came from (one of the questions mentioned in the ticket). I reached out to Zach M. to ask about it, he shared an email thread from around the time client_id was originally introduced - the id value was given to us from USPS in order for Login to be authenticated as a user for the USPS apis. I'll add a comment in the ticket with that info so we have a place to refer back to if the question pops up again.

Edited to include:
I see you have a comment already in the jira ticket with the historical information on where client_id came from, so I won't add to it afterall.