[drrtuy]

MDEV-4167 Register HLL functions and harden HLL value validation

What

Makes the HLL scalar and aggregate functions usable by registering them as
function plugins in type_hll.so. Also closes gaps where invalid or empty
values could reach an HLL column.

Key changes

• plugin.cc: add Create_func_hll<T, ARG_COUNT, AGGREGATE> builder and register
  HLL_EMPTY, HLL_CARDINALITY, HLL_CARDINALITY_ROUND, HLL_UNION,
  HLL_FORMAT_VERSION, HLL_INFO, HLL_BUILD_AGG, HLL_UNION_AGG as
  MariaDB_FUNCTION_PLUGINs; build item_hllfunc.cc into the plugin
• Field_hll::reset(): implicit default is HLL_EMPTY (0x11) instead of an
  empty string; .frm default record keeps Field_blob::reset()
• Field_hll::store(): on a malformed value, clear the field via
  Field_blob::reset() and return an error
• MTR: INSERT IGNORE does not turn an invalid HLL into a warning;
  LOAD DATA result no longer contains a build-specific path
• Update copyright headers

How to test

./mysql-test/mtr --suite=type_hll
./plugin/type_hll/hll-t

Based on top of #5573