feature: allow preloaded certs with prefork

[lublak]

Description

Currently I have a function that generates or loads these keys according to different conditions.
Now I would have to save them temporarily as a file to be able to use the full functions of fiber.
The added functions allow me to specify the already loaded keys.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• For new functionalities I follow the inspiration of the express js framework and built them similar in usage
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - /docs/ directory for https://docs.gofiber.io/
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• If new dependencies exist, I have checked that they are really necessary and agreed with the maintainers/community (we want to have as few dependencies as possible)
• I tried to make my code as fast as possible with as few allocations as possible
• For new code I have written benchmarks so that they can be analyzed and improved

Commit formatting:

Oh! Its too late :c should i recreate my pull request?

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[gaby]

Can you add unit-tests to the new exported functions?

[lublak]

Can you add unit-tests to the new exported functions?

does it make sense? I mean there are already unit tests for ListenMutualTLS and ListenTLS. And ListenMutualTLS/ListenTLS calls ListenMutualTLSWithCertificate/ListenTLSWithCertificate with already loaded file. If i create a unit test for ListenMutualTLSWithCertificate i just copy the content from ListenMutual to a unit test.

[gaby]

I think having an exported function without tests will lower the coverage % 🤔

[lublak]

@gaby i created tests for it.
and true. Unit tests unfortunately do not know that it was actually already tested with other function.

[gaby]

LGTM

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