Since Provider instances do not know the index or type for which they are responsible, it's helpful for the ProviderRegistry to include that information when returning multiple providers.
This introduces a registry service for persistence providers.
Previously, tagging one or more provider services would cause AddProviderPass to clobber the default providers created by the bundle's extension class. Now, the extension class tags its created providers and allows them to be registered via RegisterProvidersPass just like custom providers.
BC break: Custom providers tagged "foq_elastica.provider" must now specify a "type" attribute on their tag. An "index" attribute is optional (the default ES index will be used by default).
