Containerize ts-bridge, remove GAE dependencies, and make it deployable in K8S

ts-bridge is currently implemented as GAE application, which is fine for most of the potential users, but we've already faced with challenges trying to deploy ts-bridge in environments that are included into [Google VPC Service Controls](https://cloud.google.com/vpc-service-controls) service perimeters. An attempt to deploy ts-bridge with end up with an error:
{"errors":[{"code":"DENIED","message":"Request is prohibited by organization's policy. vpcServiceControlsUniqueIdentifier: "}]}.

GAE is currently [not supported by VPC service controls](https://cloud.google.com/vpc-service-controls/docs/supported-products#appengine).

The proposal is to containerize ts-bridge and remove any GAE-specific dependencies (ideally - Cloud Datastore dependencies as well). As a result it will be possible to deploy ts-bridge inside K8S cluster (e.g. in GKE) or as GAE Flex application or in any other environment that support running containers.