feat: Support mustache type export to langchain from PromptClient

[marcklingen]

Discussed in https://github.com/orgs/langfuse/discussions/1774

^{Originally posted by maxjacu April 18, 2024}

Describe the feature or potential improvement

Langchain as of recent supports (finally) mustache style variables {{var}}. Langfuse would have to implement some minor changes to export ChatPrompts correctly into that format.

ChatPromptClient
Was able to simplify conversion of get_langchain_prompt to [(msg["role"], msg["content"]) for msg in prompt].

TextPromptClient
Could just return the prompt, so no need for the helper function if mustache is used.

Additional information

No response

[marcklingen]

@maxjacu is there an upside compared to the current implementation? downside here: langchain changed this recently, thus if we change this in langfuse sdk, this does not work if used with older langchain versions