Support `UNPIVOT`

[robinicole]

Could we add support for UNPIVOT in order to be able to generate a query such as

SELECT
  a,
  metric,
  value
FROM your_table
UNPIVOT(
  value FOR metric IN (m1, m2, m3, m4)
)

I think implementation should be similar to a the GROUP_BY statement
where this

Query._from("your_table").select(
Field("a"), 
Field("metric"), 
Field("vaslue")
).unpivot(Field("value"), "metric", (table.m1, table.m2, table.m3, table.m4) 

would generate

SELECT
  a,
  metric,
  value
FROM your_table
UNPIVOT(
  value FOR metric IN (m1, m2, m3, m4)
)

[robinicole]

I am happy to work on it