[P1] Support java.math.BigDecimal and java.math.BigInteger

[DMwangnima]

java.Object, java.Time and some basic java types are implemented by idl-ref way. These types are maintained in /java directory. These PRs are related implementations:

• https://github.com/kitex-contrib/codec-dubbo/pull/62
• https://github.com/cloudwego/kitex/pull/1167

For now, we need to support java.math.BigDecimal and java.math.BigInteger. dubbo-go-hessian2 has supported encode/decode BigDecimal and BigInteger but it makes use of "github.com/dubbogo/gost/math/big".Decimal and Integer directly:

For supporting users with unified using experiences, we need to extend these two types in /java directory.

[felix021]

Btw, please help fix the problem of overwriting idl-ref.yml each time kitex is executed.

[DMwangnima]

Overwriting idl-ref.yml has been resolved by https://github.com/cloudwego/kitex/pull/1227

[DMwangnima]

This has been solved by idl-ref functionality. Waiting for doc updating.