feat(batch): support grpc streaming in xatu output

[mattevans]

Implements grpc bi-directional streaming into xatu output.

• The xatu item export now has ExportItemsViaStream alongside its existing ExportItems method.
• Stream connection is initialised, should it error, any items that fall out of the error will be pushed down the Unary path as a fallback.
• Should the stream error, attempt to reconnect will take place, with appropriate back-off up to 5 min.
• As part of this, updates to how shutdown behaviour works, handling ctx cancellation on a stream, etc.

To do:

• Streaming configuration, max connections, etc.
• Review of ctx timeouts
• Metrics
• Tests