Radius Access request to use configured src interface

[lakshmi-nexthop]

Fixes Issue: https://github.com/sonic-net/sonic-buildimage/issues/21386

Why I did it
When authentication is triggered by the DUT, source interface configured under radius needs to be used for sending access request.

How I did it
config db needs to be looked up for the source interface from the radius config and store the IP address of this interface in the pam_radius_auth file.

How to verify:

• Configured AAA authentication, configured radius with source interface as uplink port channel.
  tb1@sonic:~$ sudo config aaa authentication login radius local
  tb1@sonic:~$ sudo config radius add -s PortChannel100 -k test -p 3 10.120.20.1
• initiate ssh into the dut. this triggers radius auth request
• access request is sent out with port channel ip address as source

[linux-foundation-easycla]

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