[12.x] Online (concurrently) index creation for PostgreSQL and SqlServer

[vadimonus]

By default Postgresql and SqlServer create index using write lock (exclusive lock) on table. This makes application unusable durng migration, if index creation is long (big table), leading to downtime wich is not suitable in production.

• Postgresql offers "Concurently" option for index creation without application downtime.
• SqlServer offers "Online" option for such case.
• Latest versions of MySql and MariaDB have ALGORITHM and LOCK options, but according to documentation default option gives maximum posible concurrency (https://mariadb.com/docs/server/server-usage/storage-engines/innodb/innodb-online-ddl/innodb-online-ddl-overview#default-locking-strategy). So Changes for MySQL/MariaDB are not needed.

Currentrly creating indexex concurrently is possible only with DB::statement method. This PR adds ability to use it online() method in index definition.

Tried different names for IndexDefinition method (withoutLock, noLock, concurrently). Selected "online", as it's most widely used terminology. See https://mariadb.com/docs/server/server-usage/storage-engines/innodb/innodb-online-ddl/innodb-online-ddl-overview , https://learn.microsoft.com/en-us/sql/relational-databases/indexes/perform-index-operations-online?view=sql-server-ver16 .

Currently postgresql uses add constraint command for unique indexes, which does not support Concurently option. For such case two commands are used (create index and add constraint), as described in https://www.postgresql.org/docs/17/sql-altertable.html#SQL-ALTERTABLE-DESC-ADD-TABLE-CONSTRAINT-USING-INDEX

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