[Feature]: Allow fully custom prompts

[Danjoe4]

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Description

Why: The prompt modification options will never replicate the simplicity and customization of simply writing the prompt yourself. Users should have the choice. Allowing them to print out the prompt that would be used will give a starting point.

Suggested Solution

1. Add config option OCO_CUSTOM_PROMPT: it will ignore all other prompt-manipulating options and set the prompt to exactly what the user specifies.
2. Add an oco --show-prompt flag: prints to stdout the prompt that would be used (don't do anything)

Alternatives

No response

Additional Context

No response