feat(ai-proxy*): support resolving secrets from environment variables and secret vaults

[mikyll]

Description

This PR introduces the possibility to reference a secret in ai-proxy and ai-proxy-multi plugins, from a ENV variable or a Secret Manager service, as documented in APISIX Docs | Terminology | Secret.

This change makes it possible to avoid hard-coding the API keys in ai-proxy* plugin configuration 🙂

The changes include:

• Calling fetch_secret() in ai-proxy/base.lua to resolve secrets.
• 2 new test cases
• documentation updates

Which issue(s) this PR fixes:

Fixes #13132

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).

[Baoyuantop]

I suggest adding a test case for auth.header + $ENV://.

[Baoyuantop]

Hi @mikyll, we've just fixed some CI issues. You can merge the main branch and then run CI again.