Concrete implementation of BasePromptSelector that selects a prompt
based on a set of conditions. It has a default prompt that it returns
if none of the conditions are met.
class ConditionalPromptSelectorAbstract method that must be implemented by any class that extends
BasePromptSelector. It takes a language model as an argument and
returns a prompt template.
Asynchronous version of getPrompt that also accepts an options object
for partial variables.