Add FIRST/LAST aggregations

[smola]

• Let's consider adding FIRST/LAST aggregations.
• Present in Spark SQL, not sure if in MySQL.
• In gitbase, these are useful, for example, to get all blobs with any of their associated file names. In our case, running in gitbase-spark-connector without push down may be fine (I'm playing with that now).

[juanjux]

There doesn't seem to be implementation of these in MySQL but there are the window functions FIRST_VALUE and LAST_VALUE. Would be nice to have them in go-mysql-server anyway.

[erizocosmico]

FIRST_VALUE and LAST_VALUE require windows (not the OS), which we don't have and is significantly more complex.