Allow HTTP header contents to be marked as required

[gkanz]

Currently headers can be marked as ~required parameters in Sysl end points, but their contents can not be.

Current syntax and behaviour:

contentType <: string [~required, name="Content-Type"]

svc_client.sysl will check that the header is present in the request passed to it.

Proposed syntax and behaviour:

contentType <: string [~required, name="Content-Type", value="application/json"]

svc_client.sysl will check that the header is present in the request passed to it then:

• if it is present, check the value.
• if it is not present and required; add the header.
• if is not present and not required; no action.

[gkanz]

Duplicate of anz-bank/sysl#545

[gkanz]

Not sure my proposal is great given that this will be on every endpoint for a given app. Should probably be an app-level attribute.