Add httpate.Key(string) helper for static keys

[VojtechVitek]

Useful to create compounds with a custom prefix

[github-actions]

Benchmark Results

goos: linux
goarch: amd64
pkg: github.com/go-chi/httprate
cpu: AMD EPYC 7763 64-Core Processor                
               │ master.txt  │            pr.txt             │
               │   sec/op    │   sec/op     vs base          │
LocalCounter-4   19.81m ± 0%   19.81m ± 2%  ~ (p=1.000 n=10)

               │  master.txt  │             pr.txt             │
               │     B/op     │     B/op      vs base          │
LocalCounter-4   2.832Mi ± 0%   2.834Mi ± 0%  ~ (p=0.684 n=10)

               │ master.txt  │            pr.txt             │
               │  allocs/op  │  allocs/op   vs base          │
LocalCounter-4   121.4k ± 0%   121.5k ± 0%  ~ (p=0.671 n=10)