SQL: add NULL-specific functions

[costin]

With NULL a first class citizen, we need to add NULL specific functions, some standard, some extensions, to ease its handling. This is a meta-ticket around the functions planned:

• COALESCE SQL: Implement COALESCE function #35060
  The most generic form of null checking with fallback.
• IFNULL/ISNULL/NVL
  2 argument variant of COALESCE (IFNULL is a MySQL variant and an ODBC system function, ISNULL is a MS-SQL server variant and NVL comes from Oracle). NVL in particular needs to pay attention to the Oracle semantics.
• NULLIF
  Takes 2 arguments and returns NULL if the 2 arguments are equal, otherwise it returns the 1st one.
• <=> or null-safe equals
  This needs investigating as the standard SQL for it seems to be IS (NOT) DISTINCT FROM. H2 has a variation of it x IS (NOT) y.
  MySQL variant is the most popular though.
• GREATEST/LEAST
  Non-standard SQL but implemented by a number of DBs.

[elasticmachine]

Pinging @elastic/es-search-aggs

[matriv]

For the time being, we decided not to implement other variants of NVL (e.g.: NVL2) used by Oracle which you can see them listed here: https://oracle-base.com/articles/misc/null-related-functions#nvl2.
NVL2 for example is a particular case of COALESCE that adds some specific handling related to datatypes.

[matriv]

All functions are implemented.