impr: merge pageserver_api::models::TenantConfig and pageserver::tenant::config::TenantConfOpt (#11298)

The only difference between
- `pageserver_api::models::TenantConfig` and
- `pageserver::tenant::config::TenantConfOpt`

at this point is that `TenantConfOpt` serializes with
`skip_serializing_if = Option::is_none`.
That is an efficiency improvement for all the places that currently
serde `models::TenantConfig` because new serializations will no longer
write `$fieldname: null` for each field that is `None` at runtime.

This should be particularly beneficial for Storcon, which stores
JSON-serialized `models::TenantConfig` in its DB.

# Behavior Changes


This PR changes the serialization behavior: we omit `None` fields
instead of serializing `$fieldname: null`).

So it's a data format change (see section on compatibility below).

And it changes API responses from Storcon and Pageserver.

## API Response Compatibility

Storcon returns the location description.
Afaik it is passed through into
- storcon_cli output
- storcon UI in console admin UI

These outputs will no longer contain `$fieldname: null` values,
which de-bloats the output (good).
But in storcon UI, it also serves as an editor "default", which
will be eliminated after a storcon with this PR is released.


## Data Format Compatibility


Backwards compat: new software reading old serialized data will
deserialize to the same runtime value because all the field types
are exactly the same and `skip_serializing_if` does not affect
deserialization.

Forward compat: old software reading data serialized by new software
will map absence fields in the serialized form to runtime value
`Option::None`. This is serde default behavior, see this playground
to convince yourself:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=f7f4e1a169959a3085b6158c022a05eb

The `serde(with="humantime_serde")` however behaves strangely:
if used on an `Option<Duration>`, it still requires the field to be
present,
unlike the serde default behavior shown in the previous paragraph.
The workaround is to set `serde(default)`.
Previously it was set on each individual field, but, we do have the
container attribute, so, set it there.
This requires deriving a `Default` impl, which, because all fields are
`Option`,
is non-magic.
See my notes here:
https://gist.github.com/problame/eddbc225a5d12617e9f2c6413e0cf799

# Future Work

We should have separate types (& crates) for
- runtime types configuration (e.g. PageServerConf::tenant_config,
AttachedLocationConf)
- `config-v1` file pageserver local disk file format
- `mgmt API`
- `pageserver.toml`

Right now they all use the same, which is convenient but makes it hard
to reason about compatibility breakage.

# Refs

- corresponding docs.neon.build PR
https://github.com/neondatabase/docs/pull/470

pageserver/src/http/routes.rs

@@ -60,7 +60,7 @@ use crate::context::{DownloadBehavior, RequestContext, RequestContextBuilder};
 use crate::deletion_queue::DeletionQueueClient;
 use crate::pgdatadir_mapping::LsnForTimestamp;
 use crate::task_mgr::TaskKind;
-use crate::tenant::config::{LocationConf, TenantConfOpt};
+use crate::tenant::config::LocationConf;
 use crate::tenant::mgr::{
     GetActiveTenantError, GetTenantError, TenantManager, TenantMapError, TenantMapInsertError,
     TenantSlot, TenantSlotError, TenantSlotUpsertError, TenantStateError, UpsertLocationError,
@@ -1849,8 +1849,7 @@ async fn update_tenant_config_handler(
     let tenant_id = request_data.tenant_id;
     check_permission(&request, Some(tenant_id))?;
 
-    let new_tenant_conf =
-        TenantConfOpt::try_from(&request_data.config).map_err(ApiError::BadRequest)?;
+    let new_tenant_conf = request_data.config;
 
     let state = get_state(&request);
 
@@ -1899,7 +1898,10 @@ async fn patch_tenant_config_handler(
     tenant.wait_to_become_active(ACTIVE_TENANT_TIMEOUT).await?;
 
     let updated = tenant
-        .update_tenant_config(|crnt| crnt.apply_patch(request_data.config.clone()))
+        .update_tenant_config(|crnt| {
+            crnt.apply_patch(request_data.config.clone())
+                .map_err(anyhow::Error::new)
+        })
         .map_err(ApiError::BadRequest)?;
 
     // This is a legacy API that only operates on attached tenants: the preferred