refactor(sql): introduce DDL operations

[kszucs]

The backend classes contain a lot of code to actually compile DDL queries, these should be offloaded to the compilers themselves with possible code reuse opportunities.

I would consider to introduce DDL operations in the future and offload their compilation to the compiler possibly with some code reuse and the backend classes could get less noisy.

Originally posted by @kszucs in #7877 (comment)