support mysql8+ default values for text data types

[paulmhh]

mysql requires text type column defaults to be written as expressions https://dev.mysql.com/doc/refman/8.0/en/data-type-defaults.html

=> correct mysql8+ syntax is: DEFAULT ('text literal')

Q	A
Type	bug/improvement
Fixed issues	NA

Summary

mysql8+ does support default values for text typed columns (text, blob, json, geometry) doctrine disallows this and silently drops the default value

[derrabus]

Thank you. Please add tests.