Add search for Condition resource

We should be able to search for conditions by various properties. The most promising search properties look to be:

• patient
• patient.identifier
• patient.name
• patient.given
• patient.family
• subject
• subject.identifier
• subject.name
• subject.given
• subject.family
• encounter
• code
• clinical-status
• verification-status
• onset-age
• onset-date
• recorded-date