Adding Together API to the providers

[abirharrasse]

We make it possible for users of MetaGPT to use their together api and get access to the agentic features offered by MetaGPT.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 16.07143% with 94 lines in your changes missing coverage. Please review.

Project coverage is 62.31%. Comparing base (ab846f6) to head (2415cbc). Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
metagpt/provider/together.py	0.00%	92 Missing :warning:
metagpt/configs/llm_config.py	66.66%	2 Missing :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1474      +/-   ##
==========================================
- Coverage   62.59%   62.31%   -0.28%     
==========================================
  Files         287      288       +1     
  Lines       17589    17697     +108     
==========================================
+ Hits        11009    11028      +19     
- Misses       6580     6669      +89     

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[geekan]

We know that its api is compatible with the openai api, so in theory no new class is needed. But if I'm wrong, reopen it.