Make compression encoding configuration more malleable

[djc]

Motivation

Alternative approach to #1727, enable callers to influence compression encoding priority. Also changes the default order so that Zstd comes before Gzip if enabled (which happens when the zstd Cargo feature is explicitly enabled).

Solution

Implement an internal array while avoiding making changes to existing public API.

cc @CapCap

[tottoto]

Thanks! I feel the design and implementation is readable and extensible. I commented on small points.

[djc]

Not completely sure whether this actually solves the original issue described in #1727. IMO it would be better if the config wrapper allowed overwriting the EnabledCompressionEncodings instead of appending to it.