Make FirestoreV1 Transform bases public for worker access

[aIbrahiim]

After Java 21 bump on #39205, PostCommit Java Dataflow V1 started failing on FirestoreV1IT (batchGet, listDocuments, runQuery, partitionQuery).

The crash is IllegalAccessError on FirestoreV1$ReadTransform while the Dataflow worker loads IT DoFns and java 21 compiles BaseFirestoreIT with references to private nested types (Transform / ReadTransform) from withProjectId / withDatabaseId on the classic worker that trips getSimpleName() inside DoFnSignatures.

so the fix is making Transform, ReadTransform, and their Builders public and also make two worker DoFns package-private to match the other Firestore DoFns.

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[Abacn]

This exposes a Java17+ compatibility issue that user PTransform and DoFn defined as private static class may also fail. Testing an alternative fix in https://github.com/apache/beam/actions/runs/29358766897