Add plug-in interface to create several services based on the configuration

[d-gs]

Feature request

Expected Behavior - What were you expecting to happen?

A new IGemService for creating several services of one kind with the configuration (e.g., on #3 we can have several Feature Branch Providers (FBPs) for different JIRA instances).

Current Behavior - What happens?

Defined IGemServices are exclusively providing a single instance, which does not allow to configure something like the same FBP with different configurations at the same time.

Possible Solution

Implement a new service (IGemServiceCreator) where services of the same time are created.
The IServiceContainer should handle them and provide services as if they were independent.

Context

See #3 for an example use-case.
It can also be useful for github.com and enterprise instances running on internal networks.