Check destination connectivity and schema compatibility before starting sync

[kmacrow]

Issue

• Currently the transfer orchestration layer waits until the source is completely unloaded before checking that the destination is viable
• For small syncs this isn't a huge deal, but for large transfers you could burn a lot of time / bandwidth / cycles unloading data before failing because of an issue with the destination

Proposed solution

• Run some "preflight checks" on the destination before unloading the source data, fail early if needed
• It would be ideal to check routability, auth and schema compat before starting

Additional context

• Checking schema compatibility could be tricky because currently we don't load the source schema until the source read starts - so the orchestrator has no opportunity to pre-check against the destination
• We could factor that out into a new interface and require all sources to support a get_streams() and all destinations to support a check_streams()
• Similarly we don't have a test_connect() in the destination connector interface right now, but that would be straightforward to add - destination connection checks are actually just a tiny transfer of a mock dataset that simulates the whole process end-to-end