Added a function that returns the currently existent indexes for a table

I just added a function that returns index data. Useful when releasing a new plugin version and new indexes should be created and/or old indexes removed. Output looks like this:

```
Array
(
    [0] => Array
        (
            [Table] => qa_my_plugin_table
            [Non_unique] => 1
            [Key_name] => my_index_idx
            [Seq_in_index] => 1
            [Column_name] => id
            [Collation] => A
            [Cardinality] => 1
            [Sub_part] => 
            [Packed] => 
            [Null] => 
            [Index_type] => BTREE
            [Comment] => 
        )

    [1] => Array
        (
            [Table] => qa_my_plugin_table
            [Non_unique] => 1
            [Key_name] => my_index_idx
            [Seq_in_index] => 2
            [Column_name] => id
            [Collation] => A
            [Cardinality] => 1
            [Sub_part] => 
            [Packed] => 
            [Null] => 
            [Index_type] => BTREE
            [Comment] => 
        )
)
```