Batch insert for MSSQL

This issue doesn't fall into the issue template sorry, so I'll keep it short.

Micronaut Data disables batch insert capability for MSSQL ([code](https://github.com/micronaut-projects/micronaut-data/blob/60195d28198d90369999b02a4db4b9d4f58ef25e/data-jdbc/src/main/java/io/micronaut/data/jdbc/operations/AbstractSqlRepositoryOperations.java#L327)).

This is because MSSQL doesn't return generated primary keys for multiple inserts (I can reproduce this in plain JDBC). However, the batch insert functionality itself works fine in JDBC if you don't care about the returned IDs.

Could MSSQL batch inserts be enabled via a config flag like `micronaut.data.mssql-batch-inserts: true`? This gives users the choice.

I understand if this isn't worth the added complexity. Users can still use Micronaut for JDBC transactions and fall back to plain JDBC + JDBI or similar.

Start with data-jdbc/src/main/java/io/micronaut/data/jdbc/operations/AbstractSqlRepositoryOperations.java around the batch-insert handling at line 327. Review the plain JDBC behavior described in the issue, especially the absence of generated IDs for multiple MSSQL inserts. Done means MSSQL batch inserts can be enabled through the proposed configuration option while preserving the existing default behavior.

Written by the indexing model from the issue text.