feat: set default value of ssl_trusted_certificate to system

[Revolyssup]

Description

When testing AI plugins, we found that ssl_trusted_certificate should be set to system, otherwise APISIX will continuously report errors when accessing external AI services.

unable to get local issuer certificate

Further refactoring:
Currently two ways are used for reading configuration.

1. While generating nginx.conf file.read_yaml_conf is called
2. During lifecycle, config_local.local_conf is called which calls read_yaml_conf internally.

The problem - Schema validation for configuration happens only in local_conf so at startup, the local conf schema is not validated. This is an inconsistency.

This is especially problematic when my change uses schema to set the default to "system" because this default is only set during lifecycle and not during generation of nginx.conf.

This PR moves the schema validation inside read_yaml_conf for consistency. Now local_conf is just calling read_yaml_conf internally and adding cache.

Effect on PR of refactoring

This makes sure that consistently at all places when yaml file is read, first schema validation happens which sets the default value of trusted certificate to "system" and then later the overrides take place including the one which replaces "system" with cert paths.

Based on the discussion below, this PR also removes support for combining multiple certs
Fixes # (issue)

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)