ds/conf: use std::string_view instead to fix comparisons with literals

ds-model-test was failing on some platforms (not our ubuntu based CI) because the mocking function compared gsl::czstring (i.e. const char *) with string literal.

std::string_view is better class for key with fewer surprises.

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dss-test is a Catch v2.7.0 host application.
Run with -? for options

-------------------------------------------------------------------------------
Scenario: dssDsModel
      Given: device in zone
       Then: check dsModelZone
-------------------------------------------------------------------------------
../src/ds-model-test.cpp:1176
...............................................................................

../src/ds-model-test.cpp:1194: FAILED:
  CHECK( apartmentApi->zone(zoneId) == (dsModel::Zone{.id = zoneId, .type = "zone", .attributes = {.name = {}, .floor = "0", .orderId = 0, .submodules = {"1111111111111111111111111111111100"}, .applications = {}}}) )
with expansion:
  {"id":1,"type":zone,"attributes":{"orderId":0,"submodules":
  [1111111111111111111111111111111100]}}
  ==
  {"id":1,"type":zone,"attributes":{"floor":0,"orderId":0,"submodules":
  [1111111111111111111111111111111100]}}