plpgsql: add support for FETCH and MOVE statements

This patch adds support for the PLpgSQL FETCH and MOVE statements,
which seek and return rows from a cursor. This is handled by a builtin
function, `crdb_internal.plpgsql_fetch`, which calls into the same logic
that implements SQL FETCH and MOVE. Since it is possible to call `FETCH`
with `INTO` variables of different types, the `crdb_internal.plpgsql_fetch`
builtin takes an argument that supplies the expected column types as a
tuple of typed NULL values like this: `(NULL::INT, NULL::BOOL)`. The
actual types supplied by the cursor are coerced into the expected types.

Note that the current implementation does not support using dynamic
expressions in the FETCH/MOVE direction; only constant integer values.
Dynamic direction counts like `FORWARD x` are not allowed in SQL syntax,
but are allowed by PLpgSQL.

Informs cockroachdb#109709

Release note (sql change): Added support for PLpgSQL FETCH and MOVE
statements. Similar to SQL FETCH/MOVE statements, commands that would
seek the cursor backward will fail. In addition, expressions other than
constant integers are not yet supported for the `count` option.