Unique constraint in SQL (immudb-py)

**What would you like to be added or enhanced**
Add UNIQUE constraint to SQL

**Why is this needed**
This will allow to create Candidate Keys within a relational table. (Something which is needed quite often in many cases)

**Additional context**
An example usage of Unique constraint would be:

``CREATE TABLE table1 (id INTEGER, title VARCHAR UNIQUE, active BOOLEAN NOT NULL, PRIMARY KEY id)``

Start by tracing the SQL CREATE TABLE path described in the issue and identify how column and table constraints are parsed and enforced. Review the existing SQL tests before deciding coverage. Done means UNIQUE constraints can be declared as shown and duplicate values are rejected consistently.

Written by the indexing model from the issue text.