Allow retrieving Parquet decryption keys based on the key metadata

[adamreeve]

Follow up task to #6637, which adds initial read support for files that use Parquet modular encryption.

Currently decryption keys need to be directly specified per column, but for more flexibility and to support the key management tools API (#7256), it should be possible to provide a callback that receives the key metadata and returns the decryption key for the footer or a column.

I've made some progress towards this in a branch (https://github.com/adamreeve/arrow-rs/tree/key-retriever)