Add hostname override to AI upstream customHost setting

[andy-fong]

Description

• API change for setting hostname when customHost is use for AI Upstream

API changes

new field hostname is added to the customHost message in ai.proto

Code changes

CI changes

Docs changes

Context

Interesting decisions

Testing steps

Notes for reviewers

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[solo-changelog-bot]

Issues linked to changelog:
https://github.com/solo-io/solo-projects/issues/7831

[github-actions]

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