feat: improve linter

[bzp2010]

Description

The current linter schema is somewhat outdated and incomplete, this PR fixes these flaws and provides a JSON Schema export for the benefit of IDE users.

As far as I know, you can enable schema checking in your VSCode using a syntax like:

# yaml-language-server: $schema=https://raw.githubusercontent.com/api7/adc/main/schema.json
services: []

Note that the file schema.json is generated by the program, so the JSON Schema may not fully reflect the complete configuration schema, as some of the checksums are done at runtime by code.
I don't think you need to worry about this, execute adc lint to check the declaration file in its entirety.

In addition to this, I must say that since API7 and APISIX do not currently support the same features, and it is possible that a feature is not yet implemented on a particular platform, the configuration checked by lint does not always ensure that all fields are submited to the API, it is a best-effort effort based on the back-end API version. If you encounter a field that is not supported, the ADC or backend API may discard it or report an error.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible

[bzp2010]

Test with the following:

# yaml-language-server: $schema=https://github.com/api7/adc/blob/994a43177405c410743ac377cc198a44e252975d/schema.json

[pottekkat]

Excellent idea!