better support for nested dynamic columns

Judging from questions on #maria channel, nested dynamic columns seem to be fairly popular. But editing a column value few levels deep is very awkward. Extracting is a bit better but still inconvenient.

It would be good to have COLUMN_GET and COLUMN_ADD variants for nested dynamic columns. For example

SELECT COLUMN_GET(blob, a, b, c AS type);

as an alternative to

SELECT COLUMN_GET(COLUMN_GET(COLUMN_GET(blob, a AS blob), b AS blob), c AS type);

And

SELECT COLUMN_ADD(blob, a, b, c, value);

as an alternative to

SELECT COLUMN_ADD(blob, a, COLUMN_ADD(COLUMN_GET(blob, a AS blob), b, COLUMN_ADD(COLUMN_GET(COLUMN_GET(blob, a AS blob), b AS blob), c, value)));

Note the syntax is just an example. It's probably not possible implement it as written above (might conflict with the existing COLUMN_ADD syntax).