Instead of separate ftl-project.toml files, introduce the concept of profiles

[alecthomas]

This is very similar to how other systems work, like k8s.

• The ftl-project.toml file would change to be the location where profiles are defined.
• There would be some mechanism for switching the "default" profile, perhaps not in the ftl-project.toml file itself.
• The profile would contain the endpoint, among other things
• "Local" profiles might need some extra context in order to manage the local cluster somehow
• Split out secret storage from ftl-project.toml (see Rethink how local development state persists #2384)
• Profile name in shell prompt somehow?