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:

golang/go#66405 (comment)

[Oloruntobi1]

cc @aldas (your bio says we could @ you)

[Oloruntobi1]

Setting it seems trivial though but this will be my first time contributing so would love to hear from the maintainers first.