Bounty Endpoint: filter by fulfillment address

**User Story:**
As a developer
I want to be able to fetch completed bounties filtered by the fulfillment address
So that I can link completed bounties to wallet addresses.

**Acceptance Criteria**
GIVEN a certain wallet address
WHEN the https://gitcoin.co/api/v0.1/bounties/ endpoint is appended with the query param fulfillment_address= '0x....'
THEN the bounties fulfilled by that address are returned

**XD Links:**
N.A

**Tech Details:**

**Open Questions:**

**Notes/Assumptions:**