Support structural search mode with Compute streaming endpoint

Created by: courier-new

Note: This is NOT supporting structural find-and-replace, only supporting literal string or regex find-and-replace based on a structural search (e.g. on files that match the structural expression printf(...), replace the literal string "hello" with "goodbye").

The Compute streaming endpoint currently has regexp hard-coded as the search mode, which means you cannot use a structural search to perform a find-and-replace operation (or any other Compute command). We should accept a pattern type in the args and honor that as we do in normal search streaming.