Update buf plugin with latest v2 beta

### Summary

Buf uses 1.2.5 of betterproto package, which cannot process `optional` keywords

### What is the feature request for?

The core library

### The Problem

In our company we have a standard to use [buf](https://buf.build/) to generate bindings to proto files.
And we wanted to use [betterproto plugin](https://buf.build/community/danielgtaylor-betterproto) to generate idiomatic python code.
Unfortunately, the only version of the plugin (v1.2.5) doesn't support some "newer" protobuf features, like "optional" keyword.
This [was fixed in v.2.0.0b4](https://github.com/danielgtaylor/python-betterproto/issues/229#issuecomment-1004279896), but this release is not available in buf.

### The Ideal Solution

Can I ask to release latest beta (v.2.0.0b7 as the time of writing) as another version of [community plugin](https://github.com/bufbuild/plugins/tree/1f1ef1d4e2650f129d298941931b24f4b535b4e6/plugins/community/danielgtaylor-betterproto)?
https://github.com/danielgtaylor/python-betterproto/milestone/4 does not seem to be forthcoming.
Thanks

### The Current Solution

Rely on official buf python plugin
https://buf.build/protocolbuffers/python?version=v28.0

Beginne bei der Community-Plugin-Definition unter plugins/community/danielgtaylor-betterproto und prüfe, wie sie das betterproto-Release festlegt. Aktualisiere das Plugin auf die angeforderte Beta v2.0.0b7 und verifiziere, dass Buf die neue Version bereitstellt und dass optionale protobuf-Felder erfolgreich generiert werden.

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