fix(internal/serviceconfig): allow java for google/bigtable/v2 and set transport to grpc (#5978)

Add java to allowed languages for API path google/bigtable/v2, and set
transport to grpc to match existing code in google-cloud-java code.

For #5740

Author: zhumin8

internal/serviceconfig/sdk.yaml

@@ -276,7 +276,10 @@
 - path: google/bigtable/v2
   languages:
     - go
+    - java
     - python
+  transports:
+    java: grpc
 - path: google/chat/v1
   languages:
     - go