Secrets and Code scanning alerts API

[vlad1slav]

APIs from #549 and #526:
/repos/{owner}/{repo}/secret-scanning/alerts
/repos/{owner}/{repo}/code-scanning/alerts
/repos/{owner}/{repo}/code-scanning/alerts/{alert_number}

[vlad1slav]

@XAMPPRocky, I have questions about the src/models/code_scannings.rs file, don't know if the code in question had some logic behind it, I've added comments to the lines in question, but here is the summary:

• struct Dismisser looks like a SimpleUser, so maybe it should be removed?
• some fields mentioned in documentation as enum, but in the code they are Strings, should I convert them to enums/mention that they enums in doc comments/do not change anything there?

[XAMPPRocky]

You can remove the redundant structure and make them enums, just make sure they're non_exhaustive.

[XAMPPRocky]

Thank you for your PR!