feature/frozen-dictionary

[jamie-taylor-rjj]

Rationale for this PR

This PR enhances execution speed and thread safety by making use of the FrozenDictionary class as a cache for the calculated header values.

There is a slight penalty (but not easily perceptible) on first boot of the middleware, but all subsequent requests are now faster than when a standard Dictionary was used.

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
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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

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