Add unique indices support

[weres-sa]

YDB already has support for creating global unique indices: https://github.com/ydb-platform/ydb/blob/24.4.1/ydb/public/api/protos/ydb_table.proto#L84

That type of indices cannot be added to an existing table, according to https://ydb.tech/docs/en/concepts/secondary_indexes#limitations

Nevetheless it might be useful to allow creating tables with unique indices

[weres-sa]

Issue about unique index support is not closed yet: ydb-platform/ydb#12

It may be better to wait until it will be closed.