use rwlock for public jwk keys

[dannyboland]

Likely not relevant for most use cases but we can improve concurrency by using RwLock to allow concurrent reads of jwk keys.

Load testing

Tested with the axum-basic example.

Main branch:

> cargo run --release
> oha -m GET "http://127.0.0.1:8080/hello" -H "Authorization: Bearer $auth" -c 100 -z 10s
Requests/sec:	29876.7008

This branch:

> cargo run --release
> oha -m GET "http://127.0.0.1:8080/hello" -H "Authorization: Bearer $auth" -c 100 -z 10s
Requests/sec:   129176.3504

[trchopan]

Great!! Thank you for PR, and teaching me this optimization 🙏