[Feature] Support DelegatingCatalogExtension in Paimon SparkCatalog for unified catalog integration

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Motivation

Hi Paimon community, I'm currently working on integrating Apache Paimon into the Apache Polaris Spark catalog.
During this integration, we noticed an architectural difference in how Paimon handles **Spark catalog operations** compared to other formats like Delta and Hudi.
In unified catalogs like Polaris, Delta and Hudi achieve seamless integration by extending Spark's `DelegatingCatalogExtension`.
This allows them to delegate general catalog operations (such as namespace management and table listing) to the underlying unified catalog, while continuing to handle format-specific operations themselves.

## **The Current Challenge:**
Because Paimon's SparkCatalog does not implement this delegation model and directly manages its own warehouse, we cannot configure Polaris as a delegate catalog for Paimon. Instead, we have to rely on a complex hybrid workaround:

- We route Paimon operations through a separate catalog instance to handle operations directly against a segregated Paimon warehouse.

- After a table is created, we must manually register it as an external table back into Polaris so it appears in the unified catalog view.

## **The Request:**
Would the Paimon community be open to supporting `DelegatingCatalogExtension` (or a similar delegation architecture) in its SparkCatalog?

I'd love to hear your thoughts on the feasibility of this and if it's something that could be considered for the roadmap.

Thanks!

## **Reference:**
[Add Apache Paimon support to Polaris Spark Catalog](https://github.com/apache/polaris/pull/3723)
However, while working on the integration tests, I found out that the delegating catalog does not work.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start by reading SparkCatalog and Spark's DelegatingCatalogExtension, then review the referenced Polaris integration to understand the delegation expectations. Define how general catalog operations should delegate while Paimon-specific operations remain local, and validate the behavior with integration tests; done means Paimon works as a delegate in unified catalog integration.

Written by the indexing model from the issue text.