* Revert "Revert "🐛 requestid.Config.ContextKey is interface{} (#2369)" (#2742)"
This reverts commit 28be17f929cfa7d3c27dd292fc3956f2f9882e22.
* fix: request ContextKey default value condition
Should check for `nil` since it is `any`.
* fix: don't constrain middlewares' context-keys to strings
`context` recommends using "unexported type" as context keys to avoid
collisions https://pkg.go.dev/github.com/gofiber/fiber/v2#Ctx.Locals.
The official go blog also recommends this https://go.dev/blog/context.
`fiber.Ctx.Locals(key any, value any)` correctly allows consumers to
use unexported types or e.g. strings.
But some fiber middlewares constrain their context-keys to `string` in
their "default config structs", making it impossible to use unexported
types.
This PR removes the `string` _constraint_ from all middlewares, allowing
to now use unexported types as per the official guidelines. However
the default value is still a string, so it's not a breaking change, and
anyone still using strings as context keys is not affected.
