main/bucket: Add a callback function for ast_bucket_file objects

main/bucket: Add a callback function for ast_bucket_file objects

This patch adds a new function to the bucket API for ast_bucket_file
objects, ast_bucket_file_metadata_callback. It will call ao2_callback on
the ast_bucket_file's ao2_container of metadata, calling the provided
ao2_callback_fn callback on each piece of metadata associated with the
file.

This is particularly useful when a bucket backend has added metadata,
and a higher level API wants to be aware of/access said metadata,
without knowing for sure what the key is.

Change-Id: I96f6757717f47b650df91a437f7df16406227466