Add MiniMax AI provider

relate #3358

• add MiniMax AI provider
• minimax has different api params, link
• reuse service class ::Gitlab::Llm::ChatGlm::Completions::GenerateTestFile only
• generate new service class ::Gitlab::Llm::MiniMax::Completions::ExplainCode
• in future, service_class should be general and move out of specific ai module, prompt_class should be couple with specific ai module

relate https://jihulab.com/gitlab-cn/gitlab/-/issues/3301