Add functionality to allow custom indexes with multiple fields including meta data

[Martin-Kleinbooi-AbsaAfrica]

Hi there

It would be nice to have the ability to able to have the control over creating indexes with custom columns which includes meta data and table definition data.

Currently one can only specify either table definition data or meta data indexes.

Reason for this would for e.g. be on partitioning. When a partition is created unique indexes logic breaks because it requires both the mt_deleted field + table definition columns.

• MessageText: unique constraint on partitioned table must include all partitioning columns
• Detail: UNIQUE constraint on table "mt_doc_permission" lacks column "mt_deleted" which is part of the partition key.

[jeremydmiller]

You could do this today if the API just gave you access to the full IndexDefinition model. I think I'd rather make Marten smart enough to "know" what fields are required for partitioned tables and adds those to unique indexes automatically

Happy to have a pull request if you wanna take on the easier first suggestion up there