Add ability to pass model name to CohereEmbeddingFunction

[naynaly10]

Description of changes

Summarize the changes made by this PR.

• Improvements & Bug fixes
  • Ability to configure cohere model type for CohereEmbeddingFunction() in JS.
• New functionality
  • The CohereEmbeddingFunction() class now supports an optional parameter called cohere_model. If user does not pass a parameter, the class sets its internal model attribute to cohere's default model - "large". The generate() function uses the model attribute when making API Call to Cohere.

Test plan

How are these changes tested? JS tests (will check and edit).

Documentation Changes

Are all docstrings for user-facing APIs updated if required? Do we need to make documentation changes in the docs repository? Documentation need be updated. (Documentation need also be updated to reflect an available model parameter for OpenAIEmbeddingFunction() class)