Header Caching

[GaProgMan]

Rationale for this PR

This PR adds support for caching the calculated header values.

Each time that the middleware is called (on every request-response cycle), the following happens:

• check if the config is null
• check if the URL should be ignored
• check whether headers have been calculated previously
  • generate and cache headers
• add pre-calculated headers

PR Checklist

Feel free to either check the following items (by place an x inside of the square brackets) or by replacing the square brackets with a relevant emoji from the following list:

• ✅ to indicate that you have checked something off
• ❎ to indicate that you haven't checked something off
• ❓ to indicate that something might not be relevant (writing tests for documentation changes, for instance)

Essential

These items are essential and must be completed for each commit. If they are not completed, the PR may not be accepted.

• [ ❓ ] I have added tests to the OwaspHeaders.Core.Tests project
• [ ✅ ] I have run the dotnet-format command and fixed any .editorconfig issues
• [ ✅ ] I have ensured that the code coverage has not dropped below 65%
• [ ✅ ] I have increased the version number in OwaspHeaders.Core.csproj (only relevant for code changes)

Optional

• [ ❓ ] I have documented the new feature in the docs directory
• [ ❓ ] I have provided a code sample, showing how someone could use the new code

Any Other Information

This section is optional, but it might be useful to list any other information you think is relevant.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
OwaspHeaders.Core	87%	74%	281	✔
OwaspHeaders.Core	87%	74%	281	✔
Summary	87% (1142 / 1312)	74% (272 / 368)	562	✔