feat: Support using the stream name or alias in the context of a stream map

[edgarrmondragon]

Feature scope

Taps (catalog, state, stream maps, tests, etc.)

Description

Allow users to reference the stream name or alias in stream maps under a special key, similar to record and config:

{
    "stream_maps": {
        "countries": {
            "source_table": "__stream_name__",
            "source_key": "code",
            "source_data": "record",
            "__key_properties__": ["source_key"],
            "__else__": "__NULL__"
        }
    }
}

This should be relatively easy to add in

sdk/singer_sdk/mapper.py

Lines 331 to 334 in 2d3bf56

	names = record.copy() # Start with names from record properties
	names["_"] = record # Add a shorthand alias in case of reserved words in names
	names["record"] = record # ...and a longhand alias
	names["config"] = self.map_config # Allow map config access within transform