✨💥 #153: Allow to pass multiple model-specific param overrides

[roma-glushko]

Allowing to pass multiple model-specific chat param overrides:

• by model ID (a routing specific concept)
• by provider model name

This PR prepares a ground for #185

Changes

• Introduced a new concept/struct ChatParams that contains all param overrides for the specific modelName/modelID
• Adjusted the LangModel interface to rely on ChatParams rather than the original request schema for both sync and stream chat API
• Standardize on the chat message structure with two fields. Removed all duplicated structures
• Fixed Ollama's broken/half-backed tests

[codecov]

Codecov Report

Attention: Patch coverage is 75.59055% with 62 lines in your changes missing coverage. Please review.

Project coverage is 66.35%. Comparing base (03f1805) to head (bca8c8f).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
pkg/providers/bedrock/chat.go	54.16%	11 Missing ⚠️
pkg/api/schemas/chat_stream.go	0.00%	5 Missing ⚠️
pkg/providers/anthropic/chat.go	73.33%	3 Missing and 1 partial ⚠️
pkg/providers/ollama/chat.go	75.00%	3 Missing and 1 partial ⚠️
pkg/api/schemas/chat.go	88.88%	2 Missing and 1 partial ⚠️
pkg/providers/azureopenai/chat.go	86.95%	2 Missing and 1 partial ⚠️
pkg/providers/cohere/chat.go	72.72%	2 Missing and 1 partial ⚠️
pkg/providers/octoml/chat.go	88.88%	2 Missing and 1 partial ⚠️
pkg/providers/openai/chat.go	80.00%	2 Missing and 1 partial ⚠️
pkg/api/http/server.go	0.00%	2 Missing ⚠️
... and 14 more

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #264      +/-   ##
===========================================
- Coverage    66.98%   66.35%   -0.64%     
===========================================
  Files           79       82       +3     
  Lines         3659     3614      -45     
===========================================
- Hits          2451     2398      -53     
- Misses        1079     1094      +15     
+ Partials       129      122       -7     

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