Convert existing `X-Requested-With:` guards to `isWebServiceRequest()`

Convert existing `X-Requested-With:` guards to  `isWebServiceRequest()`

_Overview_:  `civicrm/ajax/api4` and `returnJsonResponse()` inspect the
web-request to see if it comes via AJAX/REST. If so, the call is
allowed and formatted as JSON. The patch refines the test.

_Before_: `X-Requested-With:` signals that a call is AJAX/REST.

_After_: `X-Requested-With:` still signals that a call is AJAX/REST.
Additionally, if `authx` is enabled, then some requests will be treated as
AJAX/REST based on how they are authenticated (ie `xheader`/`X-Civi-Auth:`
and `param`/`?_authx=` are AJAX/REST).