Allow custom metadata in avro files

Allow custom metadata in avro files: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). The Avro spec says the file header meta map can hold arbitrary user-defined keys alongside avro.schema / avro.codec , but fastavro doesn't seem to expose this either way:

The task

The Avro spec says the file header `meta` map can hold arbitrary user-defined keys alongside `avro.schema` / `avro.codec`, but `fastavro` doesn't seem to expose this either way:

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.