feat!: add model api read and datasource

[Gilles-Marco]

PR Dependencies

Http request authorization : #6

Changes

Added datasource litellm_model and the ReadContext for model resource

Also introduced multiple structs representing the Litellm model and model api responses.

I had to change the schema.Schema of the resource Model

Testing

resource "litellm_model" "example" {
  model_name = "example-model"

  litellm_params_custom_llm_provider = "openai"
  litellm_params_model               = "gpt-3.5-turbo"
  litellm_params_api_key             = "your_underlying_model_api_key"
  litellm_params_api_base            = "https://api.openai.com/v1"

  model_info_id  = "mysupermymodel"
  model_info_base_model = "gpt-3.5-turbo"
  model_info_tier       = "paid"
}

data "litellm_model" "example_dt" {
  model_info_id = "4d73f0a13ad754cb37f0dc0d1afb5cd08da9288ed1744560e18b84af7960af41"
}

output "litellm_model_name"{
  value = data.litellm_model.example_dt.model_name
}

Tested to read a model created by the config, it's working.
Read/update/delete/create model is working.