Allow delta staging areas to contain `.remove` and `.delete` files

[dsotirho-ucsc]

Add support for files with a .remove or .delete suffix that are allowed in a delta staging area. See DCP/2 System Design: 3.1.2 Object Naming

[melainalegaspi]

@hannes-ucsc to discuss in PL with @danielsotirhos if this is still needed.

[melainalegaspi]

@hannes-ucsc : "I agree with @danielsotirhos that this is still missing from the validator but is part of the specification. Fortunately, no one currently creates delta staging areas which reduces the urgency of this ticket."