Move peer relation code to relations/peers.py

Like for other relations, move peer relation code to a new file that can be named `src/relations/peers.py`, so we can reduce the size of the `src/charm.py` file and have a better structure, following what was established for other relations.

Start by reading the peer relation code in src/charm.py and compare it with the files for other relations. Move the peer relation code into src/relations/peers.py while following the established structure, and confirm that the charm retains the same peer relation behavior with the reduced charm.py size.

Written by the indexing model from the issue text.