feat: adds support for nginx variables in service_name param

[undying]

Description

Added support for Nginx variables in the service_name parameter for service discovery. This allows dynamic service name resolution based on Nginx variables such as $host, $http_host, and others, working with all service discovery implementations (Consul, Eureka, Nacos, etc.).

Detailed Description

Problem

The current implementation of service discovery in APISIX does not support using Nginx variables in the service_name parameter. This limits configuration flexibility as the service name must be hardcoded in the configuration, regardless of which service discovery implementation is used.

Solution

Added support for Nginx variables in the service_name parameter at the upstream level. This change enables dynamic service name resolution across all service discovery implementations. For example:

Usage Example

1. Define a variable in Nginx configuration:

map $http_host $backend {
  default echo;
  "~([^.]+).domain.local" $1;
}

2. Define routes

routes:
  -
    uri: "/*"
    host: "*.domain.local"
    upstream:
      service_name: $backend  # Nginx variable
      type: "roundrobin"
      discovery_type: "consul"  # or any other discovery type

2. Use the variable in route configuration with any service discovery:

routes:
  -
    uri: "/*"
    host: "*.domain.local"
    upstream:
      service_name: $backend
      type: "roundrobin"
      discovery_type: "consul"  # or "eureka", "nacos", etc.

In this example, if a request comes to service1.domain.local, the $backend variable will be set to service1, and APISIX will look for a service with this name in the configured service discovery system.

Backward Compatibility

The change is fully backward compatible because:

1. Existing configurations will continue to work without changes
2. The new functionality is only activated when using Nginx variables in service_name
3. The change is implemented at the upstream level, making it available to all service discovery implementations

---

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 (If not, please discuss on the APISIX mailing list first)

[moonming]

@undying please fix CI, thx