PubsubIO support for Simple Binary Encoding (SBE)

We currently have a request in Dataflow templates for Pub/Sub -\> BigQuery where the Pub/Sub topic contains SBE data. However, it doesn't seem that the PubsubIO currently supports that. Ideally, we would be able to handle cases where all we have is a schema, not precompiled classes, similar to PubsubIO#readAvroGenericRecords[1].

[1] [https://github.com/apache/beam/blob/243128a8fc52798e1b58b0cf1a271d95ee7aa241/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/pubsub/PubsubIO.java#L515](https://github.com/apache/beam/blob/243128a8fc52798e1b58b0cf1a271d95ee7aa241/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/pubsub/PubsubIO.java#L515)

Imported from Jira [BEAM-12298](https://issues.apache.org/jira/browse/BEAM-12298). Original Jira may contain additional context.
Reported by: zhoufek.

Start in sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/pubsub/PubsubIO.java around the referenced readAvroGenericRecords API. Compare that entry point with the requested schema-only SBE support, then identify the relevant PubsubIO reading path and document what behavior would demonstrate completion.

Written by the indexing model from the issue text.