feat(vision): add support for proper file and data image URLs

[Schuwi]

Currently when using vision models, sending image data as base64 in proper URL format (i.e. "url": "data:image/png;base64,iVBORw0K..." in JSON request) fails with a 500 error response on the API. I encountered this problem when using LibreChat with mistral.rs.

This PR fixes this issue by implementing proper URL parsing.

Backwards-compatibility is maintained by parsing naked file paths (e.g. "url": "some/file/path.png") and naked base64 input (e.g. "url": "iVBORw0K...") the same as before.

The new parser logic uses the url and data-url crates.
Unit tests have also been added.

[github-actions]

Code Metrics Report

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 YAML                    2           21           19            2            0
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 (Total)                            273          201           32           40
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 Markdown               29         2065            0         1570          495
 |- BASH                 5          101           98            0            3
 |- JSON                 1           12           12            0            0
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 |- Rust                 6          408          365           19           24
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 (Total)                           2753          620         1589          544
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 Rust                  198        62204        56501         1135         4568
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 (Total)                          63150        56514         2016         4620
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 Total                 315        67537        59304         2780         5453
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[EricLBuehler]

Hi @Schuwi! Thanks for the PR, it looks really great.

One thing I noticed is that there is no update to the code in mistralrs-pyo3, which also parses URLs (here). The only problem I see which prevents using the new parse_image_url function there is that the pyo3 code isn't async. Perhaps you could duplicate the function there but without async?

[EricLBuehler]

Thank you!