sock/ssl: Add psk_key and psk_identity options to spdk_sock_impl_opts

Note, this change only sets defaults for the ID/KEY,
more specific use cases like NVMe/TCP may set the ID and KEY on a per connection basis.

Also simplify PSK identity string, that isn't NVMe focused.
NVMe libraries using this will need to construct more complicated
identity strings and pass them to the sock layer.

Example:
  rpc.py sock_impl_set_options -i ssl --psk-key 4321DEADBEEF1234
  rpc.py sock_impl_set_options -i ssl --psk-identity psk.spdk.io

  ./build/examples/perf --psk-key 4321DEADBEEF1234 --psk-identity psk.spdk.io

  ./build/examples/hello_sock --psk-key 4321DEADBEEF1234 --psk-identity psk.spdk.io

Change-Id: I1cb5b0b706bdeafbccbc71f8320bc8e2961cbb55
Signed-off-by: Boris Glimcher <Boris.Glimcher@emc.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/13759


Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Mellanox Build Bot