Add Rust CPSP ProcessState options to chrome://flags.

This CL turns the ChildProcessSecurityPolicyRust flag from a
parameter-based feature flag to a multi-value choice flag. This allows
one flag to control both the main Rust-based CPSP feature and the new
ProcessState-specific sub-feature.

Change-Id: Iad89c0772292a5dc89501ceb61ceb3d5e29a2884
Bug: 522872468
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7960564
Commit-Queue: Liam Brady <lbrady@google.com>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1651093}