feat: support custom apisix.yaml path

[Siya-05]

Description

This PR adds support for specifying a custom apisix.yaml path when running APISIX in standalone mode.

Previously, APISIX only allowed overriding the path to config.yaml using the --config option, while the location of apisix.yaml was fixed under the default conf/ directory. This made it difficult to use a custom standalone configuration file.

This PR introduces:

• A new CLI option: --apisix-yaml
• Support for the APISIX_YAML_PATH environment variable
• Support for resolving a customized apisix.yaml path before falling back to the default location
• A regression test covering the customized apisix.yaml path

Which issue(s) this PR fixes:

Fixes #13338

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)