Denial of service via unbounded map allocations in github.com/iskorotkov/avro/v2 and github.com/hamba/avro/v2

The Avro map decoder accepts attacker-controlled block-element counts from the wire format and grows the destination map without enforcing an upper bound. A producer can declare an arbitrarily large map (in one block, or chunked across many sub-limit blocks) and exhaust process memory until the OOM killer fires.

The fix introduces Config.MaxMapAllocSize with cumulative enforcement across block boundaries. The new limit is opt-in: the field defaults to zero, which preserves the previous unbounded behavior for backward compatibility. Upgrading to v2.33.0 alone does not mitigate the issue; consumers of untrusted Avro data must explicitly set MaxMapAllocSize on their avro.Config.