contrib/gorilla/mux: Add WithStatusCheck()

[winebarrel]

What does this PR do?

Add WithStatusCheck() to contrib/gorilla/mux so that the function can be passed to httptrace.ServeConfig.IsStatusError.

Motivation

• Fixes contrib/internal/httptrace: proposal: Add option to ignore setting errors for 5xx HTTP status codes #2390
• I imitated the following code:
  • https://github.com/winebarrel/dd-trace-go/blob/4c27262402d0c5db2f1edbe9b274c3b902b55026/contrib/go-chi/chi/option.go#L95
  • https://github.com/winebarrel/dd-trace-go/blob/4c27262402d0c5db2f1edbe9b274c3b902b55026/contrib/gofiber/fiber.v2/option.go#L87
  • https://github.com/winebarrel/dd-trace-go/blob/4c27262402d0c5db2f1edbe9b274c3b902b55026/contrib/labstack/echo.v4/option.go#L121

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

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