Add support for error pages for VS/VSR

[Rulox]

Proposed changes

This PR introduces the error_page directive to use with VS/VSR for circuit breaking.

Changes

This PR introduces changes in how the NGINX configuration is generated using the templates. Prior to this PR, the internal redirections were done by using the error_page directive. However, this would prevent the usage of error_pages by the user, this has been changed to use the rewrite directive:

    location {{ $l.Path }} {
        rewrite ^ {{ $l.Destination }} last;
    }

instead of

    location {{ $l.Path }} {
        error_page 418 = {{ $l.Destination }};
        return 418;
    }

This change affects to the locations used for internal redirect. Before, they were named locations. But because rewrite directive won't work with named locations, regular locations need to be used. However, to prevent these locations to be accessible, the internal directive has been added.

Additionally, in order to avoid conflicts with locations defined by the user, a prefix internal_location_ will be used for these locations. This will make the locations look like this:

location /internal_location_my_location {
   internal;
   // location config
}

Instead of:

location @my_location {
   // location config
}

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[pleshakov]

@Rulox 👍

before merging, please fix a small problem in the docs

[lucacome]

Looks good @Rulox 👍