feat: upgrade sgid sdk

[kschiew]

Problem

sgID did a major release for its TypeScript SDK recently and will deprecate support for v1 SDK (which is currently used by FormSG) by the end of the year.

Solution

• Bumped major version of sgid-client to v2.
• Bumped major version of Mockpass to v4.x which is compatible to v2 of sgid-client.
• Code changes for Authenticated form via sgID flow.
• Code changes for Authenticated form with myInfo fields via sgID flow.
• Code changes for OGP officer-only login via sgID flow.

Breaking Changes

• Yes - this PR contains breaking changes
  • The code changes for the sgID flow is not backwards compatible, and should only work on snapshots where sgid-client version is >= 2.0.0.
• No - this PR is backwards compatible

Improvements:

• Introduced Proof Key for Code Exchange (PKCE) for sgID integration.

Tests

• Created unit tests for test paths where code_verifier cookie is missing.
• Updated existing test cases based on the breaking changes.
• Manually tested against sgID's staging environment:
• Log in to FormSG via the /ogp-login route. This should work correctly.
• Create and submit a form with SGID authentication. This should work correctly.
• Create and submit a form with SGID MyInfo. This should work correctly.

Deploy Notes

New environment variables:

• SGID_HOSTNAME in docker-compose.yml : updated accordingly to Mockpass' changes. No changes required for staging/prod env vars.

New dependencies:

• @opengovsg/sgid-client : Bumped from ^1.0.4 to v2.0.0.
• mockpass: Bumped from v3.1.3 to v4.0.4.

[justynoh]

Thanks so much for doing this @kschiew ! Generally LGTM with just a few small comments