Proxy samples: Add support for running with security

Proxy samples: Add support for running with security

1. Add Persistent Storage Handlers in proxy_main.c
   and proxy_client.c.
2. Add svr db files in coap proxy samples directory and
   use them in proxy_client and proxy_main for security
   credentials and ACLs.
3. Include OC_SECURE flag while initiating proxy request
   from client.

Change-Id: Iec7ea2f3a30c2777b86064e3726db25587176c21
Signed-off-by: Veeraj Khokale <veeraj.sk@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18129
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Uze Choi <uzchoi@samsung.com>