support all operators in vars field

[juzhiyuan]

When creating a Route, we could use the vars field to allow more advanced match rules.

There have 2 things we could improve:

• For each operator, we could support different actions on the value field. e.g when using the IN operator, we could use an Ant Design component that allows input values by clicking the Enter key.
• Check if we are missing some operators from https://github.com/api7/lua-resty-expr

Documentation: https://apisix.apache.org/docs/apisix/2.7/admin-api/#request-body-parameters