feat: replace special variables in instructions presets

[owengo]

Pull Request Template

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LibreChat-AI/librechat.ai#132

Summary

Following discussion at: #3713

This is a proposal to replace the specials variable: {{current_date}} and {{current_user}} in the presets before sending the request to backed.
The evaluation is done client-side, by the same function as for the custom prompts.

Change Type

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• [x ] New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Testing

Please describe your test process and include instructions so that we can reproduce your test. If there are any important variables for your testing configuration, list them here.

Test Configuration:

Configure a preset with custom instructions like this:

"You are a helpful assistant. Today is {{current_date}}. You are talking with {{current_user}}"

The endpoint will know the current date and the user it's chatting with, like for custom prompts.

Checklist

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• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have made pertinent documentation changes
• My changes do not introduce new warnings
• A pull request for updating the documentation has been submitted.