[services][identity] add rustfmt file to the cargo project

Summary:
overrides the default of 4-space tabs when cargo fmt is run. 2-space tabs are the norm in the comm codebase.

Depends on D3569

Test Plan: ran cargo fmt and confirmed that tabs were 2 spaces

Reviewers: jimpo, karol-bisztyga, palys-swm

Reviewed By: jimpo, karol-bisztyga, palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3579