Support binary image URLs without copying image data

[lasse-it]

Describe the change
This PR allows memory allocation to be optimised when providing a binary image in a ChatCompletionRequest.
It allows for providing the raw image bytes by reference, instead of having to copy the base64 encoded string.

Provide OpenAI documentation link
No API documentation is relevant.

Describe your solution
The solution makes it possible to provide any type of image URL and implements a BinaryImageURL struct that using MarshalJSON converts the raw image bytes to JSON in the most memory-effective way.

Tests

I've tested the changes using a memory profiler. I've profiled the memory allocation using a base64 encoded image URL and using the BinaryImageURL.

Using a 20mb picture, a 31,7% or 74 mb decrease in memory allocation can be seen for the provided tests.

Additional context

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 98.74%. Comparing base (774fc9d) to head (aa19584).
Report is 101 commits behind head on master.

Files with missing lines	Patch %	Lines
chat.go	73.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #853      +/-   ##
==========================================
+ Coverage   98.46%   98.74%   +0.28%     
==========================================
  Files          24       26       +2     
  Lines        1364     1437      +73     
==========================================
+ Hits         1343     1419      +76     
+ Misses         15       10       -5     
- Partials        6        8       +2     

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