[8.x] Fulltext index for PostgreSQL

[tpetry]

As promised on Twitter, I want to help with the PostgreSQL support for fulltext search.

But as fulltext searching is no SQL standard, every database has an entirely different implementation. The most significant difference is that PostgreSQL fulltext searching is language-dependent because the stemming algorithms are always only designed for a specific language. Also, adding a fulltext index for multiple columns is different. The vectors for multiple columns need to be combined for the index creation. But when querying the database the exact same syntax for combining the vectors needs to be used for the query or the index will not be used. As there is no query builder method to build these conditions, I restricted the usage to a single column for the moment. With an official way to query the index the restriction could be lifted.

So the following changes had to be made:

• An optional language parameter was added to Blueprint::fulltext()
• The PostgreSQL driver will fail if no language was provided
• The PostgreSQL driver will fail when trying to add a fulltext index with multiple columns