[upstream] Support traffic distribute by request content/header

[simon-cj]

Is this a BUG REPORT or FEATURE REQUEST?:

Uncomment only one, leave it on its own line:

/kind bug /kind feature

What happened: Now Selon Core supoort traffic distribute by weight only, we will push it to implement traffic distribute by request content/header

What you expected to happen:

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

[simon-cj]

upstream issue: https://github.com/SeldonIO/seldon-core/issues/2379