[FR] Add support for Command R+ by Cohere

[svilupp]

It would be great to support Cohere platform fully. We already use it for ranking in RAGTools.

There is an exciting new model "command-r-plus" that we want access to: https://docs.cohere.com/docs/models

TODO:

• Validate cohere_api implementation
• Add a new schema for Cohere models, eg, AbstractCohereSchema and corresponding file src/llm_cohere.jl
• Add aigenerate + aiembed implementation for Cohere models: https://docs.cohere.com/reference/chat
• Test all new functionality