[etcd-cpp-apiv3] fix find cpprestsdk when build core only

[waruto210]

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etcd-cpp-apiv3 use find_dependency(cpprest), and that will return early if cpprestsdk is not found, which results in etcd-targets not being properly imported when the async feature is not enabled.

[Mengna-Li]

Waiting for etcd-cpp-apiv3/etcd-cpp-apiv3#292.

[vicroms]

This is the incorrect fix. Instead, the patch should only call find_dependency when the async feature is enabled.

if (NOT @BUILD_ETCD_CORE_ONLY@)
  find_dependency(cpprestsdk)
endif()

[waruto210]

thanks for the reminder

waruto210 did what was requested

[BillyONeal]

Upstream has been notified and this is a minor build targeted bugfix so I think this is OK to land. Thanks for your contribution!