Support JWT key reload in pageserver (#5594)

## Problem

For quickly rotating JWT secrets, we want to be able to reload the JWT
public key file in the pageserver, and also support multiple JWT keys.

See #4897.

## Summary of changes

* Allow directories for the `auth_validation_public_key_path` config
param instead of just files. for the safekeepers, all of their config options
also support multiple JWT keys.
* For the pageservers, make the JWT public keys easily globally swappable
by using the `arc-swap` crate.
* Add an endpoint to the pageserver, triggered by a POST to
`/v1/reload_auth_validation_keys`, that reloads the JWT public keys from
the pre-configured path (for security reasons, you cannot upload any
keys yourself).

Fixes #4897

---------

Co-authored-by: Heikki Linnakangas <heikki@neon.tech>
Co-authored-by: Joonas Koivunen <joonas@neon.tech>

libs/utils/src/http/endpoint.rs

@@ -1,4 +1,4 @@
-use crate::auth::{Claims, JwtAuth};
+use crate::auth::{Claims, SwappableJwtAuth};
 use crate::http::error::{api_error_handler, route_error_handler, ApiError};
 use anyhow::Context;
 use hyper::header::{HeaderName, AUTHORIZATION};
@@ -389,7 +389,7 @@ fn parse_token(header_value: &str) -> Result<&str, ApiError> {
 }
 
 pub fn auth_middleware<B: hyper::body::HttpBody + Send + Sync + 'static>(
-    provide_auth: fn(&Request<Body>) -> Option<&JwtAuth>,
+    provide_auth: fn(&Request<Body>) -> Option<&SwappableJwtAuth>,
 ) -> Middleware<B, ApiError> {
     Middleware::pre(move |req| async move {
         if let Some(auth) = provide_auth(&req) {