docs: add RESTCatalog-specific headers table (#1446)

Author: EternalDeiwos

mkdocs/docs/configuration.md

@@ -253,6 +253,12 @@ catalog:
     header.content-type: application/vnd.api+json
 ```
 
+Specific headers defined by the RESTCatalog spec include:
+
+| Key                                  | Options                               | Default              | Description                                                                                                                                                                                        |
+| ------------------------------------ | ------------------------------------- | -------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `header.X-Iceberg-Access-Delegation` | `{vended-credentials,remote-signing}` | `vended-credentials` | Signal to the server that the client supports delegated access via a comma-separated list of access mechanisms. The server may choose to supply access via any or none of the requested mechanisms |
+
 ### SQL Catalog
 
 The SQL catalog requires a database for its backend. PyIceberg supports PostgreSQL and SQLite through psycopg2. The database connection has to be configured using the `uri` property. The init_catalog_tables is optional and defaults to True. If it is set to False, the catalog tables will not be created when the SQLCatalog is initialized. See SQLAlchemy's [documentation for URL format](https://docs.sqlalchemy.org/en/20/core/engines.html#backend-specific-urls):