Add address metadata APIs and a bridged sync engine to autofill

[DimiDL]

Firefox Desktop is moving its address storage to this component and needs two things that are not currently exposed.

Storage APIs for importing records already persisted elsewhere - add_address_with_meta, add_many_addresses_with_meta, update_address_with_meta and add_many_address_tombstones - taking a caller-supplied guid, timestamps and change counter. The bulk variants run each record in a savepoint so one bad record neither aborts the migration nor leaves a row behind. Carrying the change counter means update_internal_address now takes a CounterUpdate of Increment, Leave or Set(i64).

A bridged sync engine, so Desktop's Sync framework can drive address sync: Store::addresses_bridged_engine() exposes the address engine the crate already has, which only implemented sync15::SyncEngine and so was unreachable from Desktop's mozIBridgedSyncEngine.

Nothing on mobile calls any of this; it is additive for the Desktop migration.

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[jo]

This looks good, I esp. like with_savepoint.

I think we could go without support for sync_change_counter update via AddressMeta because when this is used during import (where I think it is designed for) the local mirror will be always empty and the record is transferred anyway.

Note that this will race with #7483 which greatly simplifies the sync bridge code.