R1:9bea00c159b6

Add harbormaster.buildplan.search api method

Summary: This revision adds a conduit search method for build plans. Other api methods (eg: `harbormaster.build.search`) support build plan phid's as a constraint, but they weren't exposed anywhere, so this provides a way to fetch them.

Test Plan:
Used the api console to run some searches. Output:
```
{
"data": [
{
"id": 1,
"type": "HMCP",
"phid": "PHID-HMCP-q2c25wvegzdkxs7gzor6",
"fields": {
"name": "my build plan"…