Move Impala operation registry to base_sql backend

[datapythonista]

For historical reasons, some code shared among backends live in the Impala backend. An important part of the shared code has been moved to a new base_sql backend in #2449.

What is pending to move in a similar way is the operation registry (and its dependencies). Each backend has a registry of the operations it defines. And many backends are using the impala one to then overwrite what it's different.

What we should do is to move the common operations to base_sql. The registry dictionary, and also the functions that define the operations such as unary, reduce... which need to get the heading underscore removed, since they'll become public.. And leave in Impala the operations that are specific to Impala (using the moved registry as a base, as the other backends are doing).

[matthewmturner]

/take

[matthewmturner]

this can be closed. sry i didnt link to my PR.

[datapythonista]

Closed via #2469