PG index options handle CONCURRENT option separately

[abbyssoul]

• [ x] Do only one thing
• [x ] Non breaking API changes
• [x ] Tested

What did this pull request do?

PR adds a check for usage of .Options field when creating indexes in PG.
Based on the feedback for the previous PR - PG Migrator allows the use of .Options to specify extra index creation options such as concurrently.
Previous PR broke that feature.
This is a fix.

User Case Description

When using Postgres driver with Gorm, users can opt to provide extra directives when creating an index.
One such PG-specific directive is to create an index ["concurrently"] (https://www.postgresql.org/docs/current/sql-createindex.html#SQL-CREATEINDEX-CONCURRENTLY). Another PG driver-specific option is to specify whether for a unique index, null values should be considered distinct or not (https://www.postgresql.org/docs/current/sql-createindex.html)
Users require a mechanism to provide these extra options for index creation. For this purpose, 'gorm options' are used: https://gorm.io/docs/indexes.html

Previous PR introduced a bug where if PG user would specify gorm:"index:,option:CONCURRENTLY" option, it would result in ill-formed index because the term 'CONCURRENTLY' will be used twice.
With this PR, users can use "CONCURRENTLY" created index option OR NULLS NOT DISTINCT option, but not both.