add UT for gRPC Encoder/Decoder

[slievrly]

• I have searched the issues of this repository and believe that this is not a duplicate.

Ⅰ. Issue Description

https://app.codecov.io/gh/apache/incubator-seata/tree/2.x/core%2Fsrc%2Fmain%2Fjava%2Forg%2Fapache%2Fseata%2Fcore%2Frpc%2Fnetty%2Fgrpc

Ⅱ. Describe what happened

If there is an exception, please attach the exception trace:

Just paste your stack trace here!

Ⅲ. Describe what you expected to happen

Ⅳ. How to reproduce it (as minimally and precisely as possible)

1. xxx
2. xxx
3. xxx

Minimal yet complete reproducer code (or URL to code):

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• JDK version(e.g. java -version):
• Seata client/server version:
• Database version:
• OS(e.g. uname -a):
• Others: