added services secrets directory to gitignore

Summary: we need a key for the identity service to work in the context of cargo run server. ignoring it so we don't have to stash it or recreate it switching between branches

Test Plan: discuss with varun/ashoat if there are any red flags to this change

Reviewers: varun

Reviewed By: varun

Subscribers: jon, abosh, tomek, atul, ashoat, varun

Differential Revision: https://phab.comm.dev/D5293