improve index support for mssql dialect

  - bring addIndexQuery in line with abstract dialect
  - use an improved method for showing indices with sp_helpindex
  - actually use handleShowIndexQuery (bringing us closer to the
    abstract dialect for formatResults)
  - temporarily disable tests for a FULLTEXT index

**NOTE** In switching the show constraint query to use sp_helpindex, I identified that we do not actually properly support schemas with this dialect yet. I'm more just leaving this here so I remember to fix it