remote_storage config: move handling of empty inline table {} to callers (#8193)

Before this PR, `RemoteStorageConfig::from_toml` would support
deserializing an
empty `{}` TOML inline table to a `None`, otherwise try `Some()`.

We can instead let
* in proxy: let clap derive handle the Option
* in PS & SK: assume that if the field is specified, it must be a valid
  RemtoeStorageConfig

(This PR started with a much simpler goal of factoring out the
`deserialize_item` function because I need that in another PR).

test_runner/fixtures/neon_fixtures.py

@@ -1167,7 +1167,9 @@ class NeonEnv:
             if config.auth_enabled:
                 sk_cfg["auth_enabled"] = True
             if self.safekeepers_remote_storage is not None:
-                sk_cfg["remote_storage"] = self.safekeepers_remote_storage.to_toml_inline_table()
+                sk_cfg[
+                    "remote_storage"
+                ] = self.safekeepers_remote_storage.to_toml_inline_table().strip()
             self.safekeepers.append(Safekeeper(env=self, id=id, port=port))
             cfg["safekeepers"].append(sk_cfg)