fix checkpoint_timeout serialization in TenantConf

Without this change, when actually setting this conf opt, the tenant
would become Broken next time we load it.
Why?
The serde_toml representation that persist_tenant_conf would write out
would be a TOML inline table of `secs` and `nsecs`.
But our hand-rolled TenantConf parser expects a TOML string.

I checked that all other `Duration` values in TenantConfOpt
use the humantime serialization.

Issues like this would likely be systematically prevent by
https://github.com/neondatabase/neon/issues/3682

pageserver/src/tenant/config.rs

@@ -103,6 +103,7 @@ pub struct TenantConfOpt {
     pub checkpoint_distance: Option<u64>,
 
     #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(with = "humantime_serde")]
     #[serde(default)]
     pub checkpoint_timeout: Option<Duration>,