Add cacheless mode escape hatch across SDKs

This issue tracks reimplementing the work from stale PR #3244, which has been closed because it is too far out of date to merge directly.

Original pull request

• PR #3244: sdk/rust: escape hatch for cacheless mode
• Original author: @vis-eyth

What the original PR was trying to do

Description of Changes These changes allow SDK consumers to register a tokio::sync::mpsc::UnboundedSender for a DbConnection that directly receives all database updates instead of the ClientCache. Rationale For writing tooling the current client cache feels very inefficient - these changes allowed us to reduce memory consumption by around a factor of 8, from being able to run the tooling for one region of BitCraft Online to all of them. Batch processing all rows in a database update as well as processing them in a different thread also lead to increased performance. Expected complexity level and risk 1 - for complexity i believe this is a fairly minor change, although not completely aware of your scale. ? - for risk. Behavioral changes only exist if the channel is registered via DbConnectionBuilder::with_update_channel, currently hidden from the docs. Risk may increase signi...

Closure context

• I'm going to close this PR since we would really need to provide this as a distinct feature in all of our SDKs. I will create a ticket to track that.

Reimplementation notes

• Reimplement this work on top of current master in a new PR.
• Keep the original PR linked as historical context and as a source of useful implementation ideas where still relevant.

Start by reviewing closed PR #3244 and its cacheless-mode design, then inspect the current DbConnection, DbConnectionBuilder::with_update_channel, and DbUpdate entry points on master. Determine how the escape hatch should be exposed consistently across the SDKs; done means the feature is reimplemented for all required SDKs rather than only Rust.