Users can define the Pebble services to be executed, but they should also be able to configure certain parameters about the Pebble software which is being installed in the ROCK. The most obvious parameter is the Pebble version, i.e., users should be able to have a say on the version of Pebble that is being used as the ROCK entrypoint. See https://github.com/canonical/rockcraft/pull/59#discussion_r954192413 for more details --- edit: Additionally, Rockcraft should know how to handle (check/support/deny) the user definitions of custom Pebble layers. Example: the user defines the PEBBLE_LAYERS_PATH and inner layer files within a custom part, conflicting with the rockcraft-defined base Pebble layer from the `services` field (see related discussion in https://github.com/canonical/rockcraft/pull/205#discussion_r1154090694)