fr: Add a customizable create index filter prefix

[jvliwanag]

Looking at the first part of the create index on some databases:

mysql> CREATE [UNIQUE | FULLTEXT | SPATIAL] INDEX index_name

sqlserver CREATE [UNIQUE] [CLUSTERED | NONCLUSTERED] INDEX index_name

spanner CREATE [ UNIQUE ] [ NULL_FILTERED ] INDEX [ IF NOT EXISTS ] index_name

spanner search CREATE SEARCH INDEX index_name

While we already have support for unique, I'm looking for support for the other keywords:

• mysql: unique, fulltext, spatial
• sqlserver: clustered, nonclustered
• spanner: null_filtered, search

I can contribute this feature with some guidance. Looking to add:

• CreateIndexBuilder: modifyPrefix(prefix: string)
• CreateIndexNode: prefix
• Add this to the default dialect after unique