Add URL prefix to pprof middleware

[gmlewis]

Description

In systems with multiple ingress endpoints all sharing the same load balancer and base URL, it is common to place servers behind a URL prefix. To facilitate this, this PR allows the pprof middleware to define a URL Prefix during configuration so that the endpoints are served on an appropriate URL.

Type of change

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• 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

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[ReneWerner87]

@gmlewis

• please update the docs in https://github.com/gofiber/docs for the middleware documentation

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