demo: an easy to use catalog loader

[Xuanwo]

Which issue does this PR close?

Part of #1254

What changes are included in this PR?

This PR introduces a catalog loader trait for Iceberg. I’ve also implemented it for the REST catalog to demonstrate its functionality.

I chose this approach to make integration with external tools, such as iceberg-playground by @liurenjie1024 or pyiceberg, easier.

Following the same pattern as pyiceberg, the loader itself only accepts a HashMap<String, String> and returns an Arc<dyn Catalog>. Users who prefer strongly-typed catalog builders can continue to use our existing APIs without any changes.

Are these changes tested?

In CI.

[Xuanwo]

In pyiceberg, we have:

class RestCatalog(Catalog):
    uri: str
    _session: Session

    def __init__(self, name: str, **properties: str):
        """Rest Catalog.

        You either need to provide a client_id and client_secret, or an already valid token.

        Args:
            name: Name to identify the catalog.
            properties: Properties that are passed along to the configuration.
        """
        super().__init__(name, **properties)
        self.uri = properties[URI]
        self._fetch_config()
        self._session = self._create_session()

It worth consider to have static property keys like we do in FileIO.

[liurenjie1024]

Suggested change

	async fn load(properties: HashMap<String, String>) -> Result<Arc<dyn Catalog>>;
	async fn load(name: String, properties: HashMap<String, String>) -> Result<Arc<dyn Catalog>>;

[liurenjie1024]

There are two problems with this approach:

1. This is not easy to use when we know the concret type of catalog. For example, when the user just wants to create a RestCatalog. It will force user to do downcast. This is useful when the catalog has some extran functionality.
2. This is not easy to use when the catalog builder has an advanced builder method, see https://github.com/apache/iceberg-rust/pull/1231/files#r2106848332

I think I can simplify the methods in my original proposal like your one, while keeping other things same, WDYT?

[Xuanwo]

Makes sense to me!