fix(xcvrd): handle port add/remove events in CmisManagerTask

This commit fixes DPB support with CMIS transceivers.

CmisManagerTask's `port_dict` must be updated according to the port add/remove events.
This commit removes the `port_mapping` field from CmisManagerTask as
`port_mapping` was mostly used just for storing `asic_id` information
and that can be simply done by `port_dict` instead.

Added a helper method `get_asic_id()` method to CmisManagerTask for
getting `asic_id` from `logical_port`.

fixes sonic-net/sonic-buildimage#18893

Signed-off-by: Wataru Ishida <wataru.ishid@gmail.com>