[identity] let clap handle parsing env var to vec

Summary:
rather than split the string ourselves, we can let clap handle this

Depends on D11250

Test Plan: tested on staging -- origins were parsed correctly and validated, only the listed origins were allowed

Reviewers: will, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11251