Fix group settings to allow external member into oss-fuzz cc groups

[ViniciustCosta]

Creating groups through the cloud identity API uses a default set of access settings. We need to change these in order for them to work as CC groups for OSS-Fuzz projects. Mainly, allowing external members to be added to the group. These changes cannot be done through the identity API, so we need to use the groups settings API (which I enabled for the clusterfuzz-external deployment).

I added some other settings as safeguard to avoid issues with these groups (as they will be in the public tracker), such as who can view/post/contact/manage the group to only owners/managers. Also, fixed some single/double quote style.

Tests

• Added unit test.
• Run locally impersonating the clusterfuzz-external service account.
  • Output: https://paste.googleplex.com/4898709642018816 (errors in the first run are expected as googleapiclient logs an error when the checking for the group and it does not exist).
  • Group settings after executing it: https://screenshot.googleplex.com/5ymqVPoFMwHYraN.png

Context: b/477964128