feat: Add providers startup flag

[mikenairn]

closes #105

Add a new flag to the controller startup that determines what providers are enabled, if none are specified the list of registered default providers is used.

$ go run ./cmd/main.go --provider google,inmemory --provider aws
2024-05-28T17:29:15+01:00       INFO    setup   init provider factory   {"providers": ["google","inmemory","aws"]}
2024-05-28T17:29:15+01:00       INFO    setup   starting manager
....

$ go run ./cmd/main.go --provider aws
2024-05-28T17:32:28+01:00       INFO    setup   init provider factory   {"providers": ["aws"]}
2024-05-28T17:32:28+01:00       INFO    setup   starting manager
....

$ go run ./cmd/main.go
2024-05-28T17:32:48+01:00       INFO    setup   init provider factory   {"providers": ["google","aws"]}
2024-05-28T17:32:48+01:00       INFO    setup   starting manager
....

The provider factory is updated to take a list of providers that are enabled and ensures that only providers in this list can be loaded via the factory methods. Providers themselves optionally register themselves as "default" providers, in the case where no providers are specified these providers are enabled in the factory.

[philbrookes]

looks good, just want to make sure I've understood why inmemory is set to a default provider.