Add file transfer size configuration

[dmlb2000]

Description

This adds a configuration option to the cart that configures the transfer size for downloading data from the archive interface and serving files through the cart.

Signed-off-by: David Brown dmlb2000@gmail.com

Issues Resolved

Fix #52

Check List

• [ ] All tests pass. See https://github.com/pacifica/pacifica-docs/blob/master/development/TESTING.md
• [ ] New functionality includes testing.
• [ ] New functionality has been documented in the README if applicable