feat: Implement "include" feature for yaml config.

[teodorescuserban]

Description

This relates to #11501

Being able to not repeat yourself and split the apisix.yaml would be a really neat feature.

[juzhiyuan]

How about using YAML Anchor syntax? I know some users are building the final YAML files in CI/CD pipelines.

[teodorescuserban]

How about using YAML Anchor syntax? I know some users are building the final YAML files in CI/CD pipelines.

I am sorry for not responding sooner, I will try and test when I am able and will provide feedback.

[Baoyuantop]

If there is still a problem, please open it again.