Use SignatureKey as auth_type for VGS connector

We need to collect a 3rd key (vault_id) apart from `username` and `password` in case of VGS for SDK use case
<think>
PR Context:
- Title: fix(connector): Use SignatureKey for VGS auth_type
- Merged: 2025-08-28T02:44:22Z
- Files changed: 1 files
- Commits: 3 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
- Change VGS auth type from BodyKey to SignatureKey

### Additional Changes

- [] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** fix(connector): Use SignatureKey for VGS auth_type (#9068)
Files to modify:


**crates/hyperswitch_connectors/src/connectors/vgs/transformers.rs**
  Modify existing code
{"issue_number": 9067, "pr_number": 9068, "commits_count": 1, "files_changed": 1, "response_length": 952, "file_count": 3, "needs_breakdown": false}