Support for HLL Functions

[carlineng]

What happens?

Malloy should support HLL functions, like those available in BigQuery

A few things that make this a bit trickier than the typical aggregate function:

1. The functions in BQ are weirdly named, e.g., HLL_COUNT.EXTRACT and HLL_COUNT.INIT. Can we just map these to Malloy-native function names like HLL_COUNT_EXTRACT and HLL_COUNT_INIT?
2. These are not supported by Postgres and DuckDB, so we need a strategy to support functions in one dialect but not another.

To Reproduce

See above.

OS:

All

Malloy Client:

All

Malloy Client Version:

All

Database Connection:

BigQuery