feat: extend chaitin-waf plugin functionalities

[lingsamuel]

There are some todos need to be done in chaitin-waf plugin.

1. add a configuration for "mode" attribute
2. add a configuration to enable/disable the real client IP
3. use lrucache to cache the var expression result (traffic-split plugin also need this)

[PalAditya]

Hi, I'd like to work on this if possible. Adding a few questions as well to make sure my understanding is correct:

• Since https://github.com/chaitin/lua-resty-t1k accepts the modes, do we want to just add it here: https://github.com/apache/apisix/blob/master/apisix/plugins/chaitin-waf.lua#L62 ? Should we change the output messages accordingly as well?

• I am guessing we can use core.request.get_remote_client_ip to get the client ip, but what would the config do? If set to disabled, should we just pass in a hardcoded ip like 127.0.0.1 in the config variable remote_addr?

• This seems to be a simple one line addition of the lrucache module - we can probably start with TTL of 300 and size 1024?