Add engine_adapter_callback

[umar-ahmed]

Description

• Adds engine_adapter_callback that is executed after the SQLAlchemy engine is constructed
• This allows developers to apply any arbitrary statements before subsequent queries are issued
• In particular, I want to be able to run SET workload = "olap"; to connect to a Planetscale DB without bumping into 100k rows returned system limits.

Related Issues

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Additional Context

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[rudolfix]

@umar-ahmed sql_database and sql_table accept Engine instance in credentials. So you can create your own, run anything on it and then pass it to dlt. OFC having this callback is more convenient because you do not need to handle credentials yourself. if you plan to continue this PR we will also need a simple test where you verify that adapter receives engine instance