feat(fault-injection): support conditional fault injection using nginx variables

[Firstsawyou]

What this PR does / why we need it:

We can add new features to the fault-injection plugin for the following reasons:
Currently, the fault-injection plugin does not support custom conditions to implement the mock function. I think we can add the request header as a conditional judgment in the request (there may be a better way) to realize the mock function.
E.g:

"vars": [
     [[ "http_mock01", "==", " api01" ]],
     [[ "http_mock02", "==", " api02" ]]
]

If vars passes the verification, fault injection is performed on the request. Otherwise, no processing is done on the request. In order to maintain backward compatibility, the fault injection operation is still performed when the vars condition is not configured.

close #2511

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[membphis]

ci failed

[Firstsawyou]

ci failed

I need to adjust the code logic and I will solve the ci problem later.

[spacewander]

I think we need to remove the vars_schema.

1. we can do the check better with expr.new when checking the schema.
2. when not/or is supported directly in the expr, this schema is outdated.
3. this schema contains many incorrect hardcoded limitations, like the maxLength of operator is 2 (not enough for has), and an expression may contain 4 elements if ! is used, etc.

[Firstsawyou]

updated.

[spacewander]

No, you don't remove it.

[spacewander]

I think just keep:

local vars_schema = {
    type = "array",
    maxItems = 20
}

is enough. The remain can be validated via expr.new.

[Firstsawyou]

Okay, let me try it.

[Firstsawyou]

updated.

[spacewander]

Should be failed to create vars expression, we haven't matched it yet.

[Firstsawyou]

updated.

[spacewander]

Better to check the expression here and add a test for it.

[Firstsawyou]

updated.

[spacewander]

No, I mean checking the expression via:

        local expr, err = expr.new(var)
        if err then
            core.log.error("failed to create vars expression: ", err)
            return nil, err
        end

[Firstsawyou]

added.