Allow "harbormaster.createartifact" to decode raw HTTP parameter types of artifact properties

Summary:
Ref T11887. This isn't a great fix but makes the method behave properly until I get around to a real fix.

In the longer term, I want to convert all of this pluggable Harbormaster/Drydock stuff (blueprints, artifacts, build plans) to use EditEngine + EditField instead of the weird mishmash of older/custom stuff it currently uses. However, this is a more involved project to execute and I'd like to be in that area of the codebase first so it gets adequate testing.

Until that happens, just put a reasonble-ish mechanism in place to let artifacts correct inbound types. This is the only artifact type and only parameter which needs casting.

Test Plan:

• Made a curl call to harbormaster.createartifact to create a URI artifact with ?...&ui.external=1.
• Before patch: type error on ui.external not being a boolean.
• After patch: artifact created successfully.

Reviewers: chad

Reviewed By: chad

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T11887

Differential Revision: https://secure.phabricator.com/D16887