Allow users to specify a download source for beats packages

The experience for developers working on issues spanning integration packages and their backing beats agent executables or fleet managed beats data sources in general is currently quite frictionful; the process involves direct interaction with the underlying docker containers to replace executables in known, but relatively obscure locations in the agent container's file system.

Fleet does provide an [option to specify agent binary downloads](https://www.elastic.co/guide/en/fleet/8.8/fleet-settings.html#fleet-agent-binary-download-settings), so it would be helpful if `elastic-package` could be configured to establish an artefact server to host agent binaries and configure fleet to point to this server. If the server were set up as a MitM proxy for https://artifacts.elastic.co/downloads/ when the artefact does not exist locally, this would make the system essentially a transparent drop location to place agent binaries.

Start with the elastic-package CLI workflow for integration packages and the Fleet agent binary download settings linked in the issue. Review how agent executables are currently obtained from https://artifacts.elastic.co/downloads/ and how Docker containers are used during development. Done means users can configure a local artifact server that Fleet uses for agent binaries, with a fallback to the upstream source.

Written by the indexing model from the issue text.