samples: bt: hids: add a non authenticated mode to the sample

The HIDs sample is currently setup with a passkey callback and requires
authenticated write and read access.

Add a sample option to disable the passkey callbacks, and automatically
set the GATT attributes as encryption required. This is a useful sample
setup as real world HID devices (mice, keyboards...) usually don't have
a passkey mechanism, and removing the callback to reproduce that setup
while not changing the GATT permission leads to automatic disconnections
for encryption failures that can be fairly hard to troubleshoot.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>