create order-search hook

[JPercival]

This PR contains a proposal for a new order-search hook which would be the first hook fired when a clinician is browsing the catalogue of a CPOE. The allows recommendations for specific orders to be made to the clinician by the decision service, or the ability to launch a SMART app that could gather additional information and then subsequently make a recommendation.

This is in contrast to order-select in which a specific order has been chosen and further details can be filled out.

The hook defines a orderType context parameter to allow only cds services that a relevant to the current type of order to be run. Hypothetically, without this additional context a clinician may be searching for Medications and get Procedures instead, since the definition of "order" is very broad. The order-select hooks is able to avoid this because the selected order that is passed has a specific type, meaning decision services that apply to MedicationRequest selections need not fire if a bundle contains ServiceRequests.

Two other designs were considered in order to achieve the context-specific functionality:

1. Defining a more specific hook that would be <orderType>-search (e.g. medication-search). This runs counter to the current direction of cds-hooks, where type-specific hooks are being retired (such as medication-prescribe).

2. Use a useContext resource. This provides a way to convey a very broad range of clinical contexts to the decision service (such as in-patient, out-patient, and the type of issue being treated) but was thought to be too abstract and general making implementation of the hook in EHRs difficult.