NimBLE/SM: Application should be able to accept or reject pairing request

- Current implementation does not have any support to accept/reject pairing request, having this support will be helpful for implementing different security levels.
- As an use-case, if the device wants to only establish Authenticated Secure Connection only connection, then any unauthenticated pairing request can be rejected.

The issue names NimBLE/SM pairing-request handling but no files or tests. Start by tracing the pairing request entry point and how application callbacks are exposed, then determine the API and state flow needed for an application decision. Done means applications can accept or reject requests, including rejecting unauthenticated pairing when authenticated secure connections are required.

Written by the indexing model from the issue text.