[BREAKING_CHANGES] refactor: change metadata type from map[string]string to map[string]any

[anilmisirlioglu]

Describe the change
We use Litellm and Langfuse and I realized that the map[string]string data type in the metadata field is insufficient to pass some trace fields (e.g. tags, trace_metadata) from the metadata. It is possible to make metadata type map[string]any for more complex data types. There is no statement in the OpenAI document stating that metadata must be of type map[string]string

Provide OpenAI documentation link
Litellm Langfuse Integration
OpenAI Metadata -> Set of 16 key-value pairs that can be attached to an object. This can be useful for storing additional information about the object in a structured format, and querying for objects via API or the dashboard.

Describe your solution
I changed the type of the metadata field in ChatCompletionRequest and ModifyMessage to map[string]any

Tests
I ran the tests locally.

Issue: -

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.88%. Comparing base (774fc9d) to head (37e834c).
Report is 87 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #944      +/-   ##
==========================================
+ Coverage   98.46%   98.88%   +0.41%     
==========================================
  Files          24       27       +3     
  Lines        1364     1786     +422     
==========================================
+ Hits         1343     1766     +423     
+ Misses         15       14       -1     
  Partials        6        6              

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