[Feature Request]: extensions/protobuf does not support google.protobuf.Struct

### What would you like to happen?

When a protobuf schema contains a Struct, constructing Beam Schema from proto schema fails with

```
IllegalArgumentException: Cannot infer schema with a circular reference. Proto Field: google.protobuf.Struct
```

Example stack trace

```
com.google.cloud.teleport.v2.common.UncaughtExceptionLogger - The template launch failed.
java.lang.IllegalArgumentException: Cannot infer schema with a circular reference. Proto Field: google.protobuf.Struct
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.getSchema(ProtoSchemaTranslator.java:174)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromSingularProtoField(ProtoSchemaTranslator.java:344)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromProtoField(ProtoSchemaTranslator.java:253)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.getSchema(ProtoSchemaTranslator.java:192)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromSingularProtoField(ProtoSchemaTranslator.java:344)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromProtoField(ProtoSchemaTranslator.java:253)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromProtoField(ProtoSchemaTranslator.java:247)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.getSchema(ProtoSchemaTranslator.java:209)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromSingularProtoField(ProtoSchemaTranslator.java:344)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromProtoField(ProtoSchemaTranslator.java:253)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.getSchema(ProtoSchemaTranslator.java:209)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromSingularProtoField(ProtoSchemaTranslator.java:344)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.beamFieldTypeFromProtoField(ProtoSchemaTranslator.java:253)
at org.apache.beam.sdk.extensions.protobuf.ProtoSchemaTranslator.getSchema(ProtoSchemaTranslator.java:209)
at org.apache.beam.sdk.extensions.protobuf.ProtoDynamicMessageSchema$DescriptorContext.(ProtoDynamicMessageSchema.java:254)
at org.apache.beam.sdk.extensions.protobuf.ProtoDynamicMessageSchema.(ProtoDynamicMessageSchema.java:69)
at org.apache.beam.sdk.extensions.protobuf.ProtoDynamicMessageSchema.forDescriptor(ProtoDynamicMessageSchema.java:92)

```

THis affects `PubsubIO.readProtoDynamicMessages`

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

### Issue Components

- [ ] Component: Python SDK
- [X] Component: Java SDK
- [ ] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [ ] Component: Beam YAML
- [ ] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [ ] Component: Website
- [ ] Component: Infrastructure
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner