Added client_secret.bcheck

[LabMC]

BCheck Contributions

• The "client_secret.bcheck" file has been created to detect for the "client_secret", "client_id", & "refresh_token" OAuth API variables, plus variants of "api-key", within front-end files. This file is placed in the other/API folder.
• Minor update to Content-Security-Policy.bcheck to look for findings in .js & .css files.

• BCheck compiles and executes as expected
• BCheck contains appropriate metadata (name, version, author, description and appropriate tags)
• Only .bcheck files have been added or modified
• BCheck is in the appropriate folder
• PR contains single or limited number of BChecks (Multiple PRs are preferred)
• BCheck attempts to minimize false positives

[PortSwiggerWiener]

Many thanks for the submission! Looks really good. Minor comments above.

[PortSwiggerWiener]

Many thanks for the revisions.

Looks good 👍

[Hannah-PortSwigger]

Looks good 👍