Server-Timings support

[VlaDexa]

This library lack support for parsing and creating Server-Timings headers. There can be multiple in a request, and one header can have multiple metrics with each metric having a name and 2 optional properties of duration, commonly represented as a number of milliseconds measured, and description.

[seanmonstar]

It's possible for other crates to define custom headers, by just implementing the Header trait.

I'm not sure we'll add server-timings directly here.