[Go][Experimental] better oneOf and anyOf implementation

[wing328]

• better oneOf and anyOf implementation (based on java jersey2-experimental approach)

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)

[LaysDragon]

I'm encounter some problem and trying to switch to latest version of 5.0.0beta2 to see if the problem is fixed. Then I noticed the implementation of oneof discriminator is quite lot different from 4.3.1 go-experimental
hmmmm,pregenerated static composed fields for every oneof type is better then original implemented interface field way(#5150)?