fix: only parse and validate apisix.yaml in cli when startup

[nic-6443]

Description

The purpose of the read_yaml_conf function is to read config.yaml, but it also parses and validates apisix.yaml. This validation only needs to be performed when the CLI module starts, and there is no need to repeatedly parse and validate apisix.yaml during runtime when try to get config.yaml:

apisix/apisix/core/config_local.lua

Line 61 in 958312f

local default_conf, err = file.read_yaml_conf()

Which issue(s) this PR fixes:

Fixes #

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)