Allow to change the PubsubClientFactory when using PubsubIO

When we use PubSub to push or pull messages, we currently have no choice of serialization implementation because PubsubIO use internaly the *_PubsubJsonClient.FACTORY_* of type _*PubsubJsonClientFactory*_ to serialize or deserialize messages.

It should be nice to be able to set a different factory (e.g. *_PubsubGrpcClientFactory_*) to decrease the size of messages (and then decrease the price of using Pubsub).

I guess It could be possible to write something like:

`PubsubIO.Write write =`
`    PubsubIO.writeMessages()`
`            .to("projects/project/topics/topic")`
`            *.withFactory(PubsubGrpcClient.FACTORY)*`
`            .withTimestampAttribute("timestamp")``;`

or 

`PubsubIO.Read read = `
`    PubsubIO.readMessages()`
`            .fromSubscription("projects/project/subscriptions/name")`
`            *.withFactory(PubsubGrpcClient.FACTORY)*`
`            .withTimestampAttribute("timestamp");`

Imported from Jira [BEAM-5676](https://issues.apache.org/jira/browse/BEAM-5676). Original Jira may contain additional context.
Reported by: lhauspie.

Start by reading the PubsubIO readMessages and writeMessages entry points and how they currently use PubsubJsonClientFactory. Compare the mentioned PubsubGrpcClientFactory integration points for both reading and writing; done means callers can select a factory and messages are serialized and deserialized through it.

Written by the indexing model from the issue text.