Consolidate content type and content encoding functions in prometheus/common

I recently added zstd support to client_golang in https://github.com/prometheus/client_golang/pull/1496 which uses an internalized version of the archived repository: https://github.com/golang/gddo/blob/master/httputil/negotiate.go

I see that https://github.com/prometheus/common/blob/2d5ba4a4a8854b224d06df286a191f710fc65785/expfmt/encode.go#L67 was recently changed to use https://github.com/munnerz/goautoneg for contentType logic in https://github.com/prometheus/common/pull/625

Would you be open to include the logic for content encoding implemented in client_go in here and replace goautoneg with ggdo/httputil (we could either consume from the archived repository or internalize it into this repository; we took the second approach for client_golang)?

This likely would help with consolidating the library for content encoding as well as content type and allow reusing parts of it in prometheus/prometheus as well (e.g. in https://github.com/prometheus/prometheus/issues/13866)

CC: @bwplotka

Start with expfmt/encode.go at the referenced contentType logic, then compare it with the content-encoding implementation in client_golang and the archived gddo/httputil/negotiate.go. Determine whether the shared logic should consume the archived repository or be internalized, and verify that content type and content encoding behavior remain compatible for the affected Prometheus components.