Use serde for RemoteStorageConfig parsing (#8126)

Adds a `Deserialize` impl to `RemoteStorageConfig`. We thus achieve the
same as #7743 but with less repetitive code, by deriving `Deserialize`
impls on `S3Config`, `AzureConfig`, and `RemoteStorageConfig`. The
disadvantage is less useful error messages.

The git history of this PR contains a state where we go via an
intermediate representation, leveraging the `serde_json` crate,
without it ever being actual json though.

Also, the PR adds deserialization tests.

Alternative to #7743 .

proxy/src/context/parquet.rs

@@ -543,7 +543,9 @@ mod tests {
         rx: impl Stream<Item = RequestData>,
     ) -> Vec<(u64, usize, i64)> {
         let remote_storage_config = RemoteStorageConfig {
-            storage: RemoteStorageKind::LocalFs(tmpdir.to_path_buf()),
+            storage: RemoteStorageKind::LocalFs {
+                local_path: tmpdir.to_path_buf(),
+            },
             timeout: std::time::Duration::from_secs(120),
         };
         let storage = GenericRemoteStorage::from_config(&remote_storage_config).unwrap();