Add custom base URL for Google VertexAI provider

[tequeter]

What specific problem does this solve?

We'd like to direct VertexAI calls to a different base URL, similar to what the OpenAI provider already allows.

Use-case : we are proxying LLM API calls through an API gateway to support per-user virtual API keys and usage reporting.

I can probably volunteer a PR if it makes it happen faster. I'll wait for your feedback before diving into Roo's codebase, though.

Additional context (optional)

This feature already has a proposed implementation in PR #4815. However, the author did not reply to requests for changes.

Here is the expected UI (copied straight from that PR) :

Roo Code Task Links (Optional)

No response

Request checklist

• I've searched existing Issues and Discussions for duplicates
• This describes a specific problem with clear impact and context

Interested in implementing this?

• Yes, I'd like to help implement this feature

Implementation requirements

• I understand this needs approval before implementation begins

How should this be solved? (REQUIRED if contributing, optional otherwise)

No response

How will we know it works? (Acceptance Criteria - REQUIRED if contributing, optional otherwise)

No response

Technical considerations (REQUIRED if contributing, optional otherwise)

No response

Trade-offs and risks (REQUIRED if contributing, optional otherwise)

No response

[roomote]

I'm exploring a potential fix for adding custom base URL support to the VertexAI provider and will open a PR to share progress.

[roomote]

I've opened PR #7900 with my attempt at addressing this issue. The implementation adds a custom base URL field for the VertexAI provider, similar to how other providers handle it. Feedback welcome!