chore: re-design nginxVars to exprs

[tokers]

Please answer these questions before submitting a pull request

• Why submit this pull request?

• Bugfix

• New feature provided

• Improve performance

• Backport patches

• Related issues

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Bugfix

• Description

• How to fix?

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New feature or improvement

• Describe the details and related test reports.

This PR renames the nginxVars field in ApisixRoute to exprs, which is more expressive, also, the subject in exprs is re-designed to be an object which contains the scope and name field.

The scope field specifies where to find the subject, it can be:

• Header - HTTP request headers
• Cookie - Cookie
• Query - Query string
• Path - URI Path

The name field specifies the subject name, which can be empty when scope is Path.

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Backport patches

• Why need to backport?

• Source branch

• Related commits and pull requests

• Target branch

[tokers]

@membphis @gxthrj Please take a look when you have time.