🚀 [Feature]: add envvar expose middleware

[mstrYoda]

Description

This pr adds environment variable expose middleware.

Fixes #2053

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• 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 - https://github.com/gofiber/docs 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

[welcome]

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[efectn]

Please update awesome-fiber and https://github.com/gofiber/fiber#-internal-middleware

[mstrYoda]

Please update awesome-fiber and https://github.com/gofiber/fiber#-internal-middleware

All done 👍

[efectn]

Dont forget adding middleware section to other READMEs

[efectn]

Check actions please. Lgtm

[mstrYoda]

Check actions please. Lgtm

I changed t.Setenv to os.Setenv to provide backward compatibility with older Go versions.

[ReneWerner87]

please check again the methods of envvar (some of them are not use and lowercase -> so not exposed )

can you please also provide a better example and link where such functionality is used in other languages or frameworks so i can better understand the use of middleware

[mstrYoda]

please check again the methods of envvar (some of them are not use and lowercase -> so not exposed )

can you please also provide a better example and link where such functionality is used in other languages or frameworks so i can better understand the use of middleware

Well, I quickly searched but could not find an exact example of this. People are trying to reimplement their middleware/components for this work. We always wonder about our env vars that contain feature flags too in our APIs.
This is why I thought that it would be good to have built-in one.

[welcome]

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