fix openai handler api_base argument

[StpMax]

Description

Finetune was blocked because of:

• if api_base not in args, and OPENAI_API_BASE not in env, then key error raised
• api_base can not be in args because it is not in white list

Fixes #issue_number

Type of change

(Please delete options that are not relevant)

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ⚡ New feature (non-breaking change which adds functionality)
• 📢 Breaking change (fix or feature that would cause existing functionality not to work as expected)
• 📄 This change requires a documentation update

Verification Process

To ensure the changes are working as expected:

• Test Location: Specify the URL or path for testing.
• Verification Steps: Outline the steps or queries needed to validate the change. Include any data, configurations, or actions required to reproduce or see the new functionality.

Additional Media:

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Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have appropriately commented on my code, especially in complex areas.
• Necessary documentation updates are either made or tracked in issues.
• Relevant unit and integration tests are updated or added.