Support setting http.Request.Pattern in router

[Oloruntobi1]

Echo’s router doesn’t set http.Request.Pattern.

Helps OpenTelemetry (http.route) and aligns with net/http. Related: open-telemetry/opentelemetry-go-contrib#6980.

Also related:

https://github.com/golang/go/issues/66405#issuecomment-2113163518