YOJ 2.5.6 (BAD deps - UNIQUE indexes)

⚠️ Please do not use this release, it has incorrect YDB Proto API version dependency (too old; 1.6.2 instead of at least 1.6.4). Use the subsequent 2.5.7 release instead.

• #88: Support UNIQUE indexes (see https://ydb.tech/docs/en/concepts/secondary_indexes#unique)
• #87: YdbRepositoryTransaction can now return YDB SDK's YdbTransaction to use with YDB SDK calls, e.g. for topics (see issue #80)