Add a non-strict version of jsonb_set

jsonb_set_lax() is the same as jsonb_set, except that it takes and extra
argument that specifies what to do if the value argument is NULL. The
default is 'use_json_null'. Other possibilities are 'raise_exception',
'return_target' and 'delete_key', all these behaviours having been
suggested as reasonable by various users.

Discussion: https://postgr.es/m/375873e2-c957-3a8d-64f9-26c43c2b16e7@2ndQuadrant.com

Reviewed by: Pavel Stehule