HTTP Endpoint: Implement RFC 3986 compatibility as optional

[sigma-software-prebid]

🔧 Type of changes

• [ ] new bid adapter
• [ ] update bid adapter
• [x] new feature
• [ ] new analytics adapter
• [ ] new module
• [ ] bugfix
• [ ] documentation
• [x] configuration
• [ ] technical debt (test coverage, refactoring, etc.)

✨ What's the context?

https://github.com/prebid/prebid-server/issues/3595

🧠 Rationale behind the change

According to RFC 3986 specification a query cannot contain symbols like [, ] or " which are allowed according Go server documentation (stored_requests/backends/http_fetcher/fetcher.go), so it was decided that default HTTP URL builder should have an option in configuration to follow RFC 3986 specification, however in order to preserve backward compatibility by default this option (stored_requests.http.use_rfc3986_compliant_request_builder) should be false.

🧪 Test plan

New functionality is covered by Test*RfcCompliant set of unit tests in stored_requests/backends/http_fetcher/fetcher_test.go.