Add support for SQL based registration of external catalogs

### Is your feature request related to a problem or challenge?

When using the Datafusion REPL as an exploration tool it can be convenient to connect to an external catalog (e.g. an Iceberg REST catalog) ad hoc using a SQL statement. At the moment this is not possible since `ATTACH ...` or `CREATE EXTERNAL CATALOG ...` are not supported. The session state also does not provide any means to register external catalog factories the same way table factories can be registered.

### Describe the solution you'd like

Add an equivalent to `TableProviderFactory` for external catalogs.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start in the DataFusion REPL and trace how ATTACH or CREATE EXTERNAL CATALOG statements would reach session state. Compare the existing TableProviderFactory registration path with the requested external catalog factory equivalent. Done means an external catalog factory can be registered and an external catalog can be created through SQL.

Written by the indexing model from the issue text.