Adding support for custom endpoint for OpenAI Conversation Component

[willvelida]

Description

Adding support for a custom endpoint for the OpenAI Conversation Component.

Issue reference

Please reference the issue this PR will close: #3808

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation / Created issue in the https://github.com/dapr/docs/ repo: dapr/docs#[issue number]

[marcduiker]

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