Fix: adapt function to translate cached prompts in place.

[HerrIvan]

Address issue #888

It enables the following functionality in order to generate a testset in a language other than English.

from ragas.testset.prompts import testset_prompts
from ragas.testset.generator import TestsetGenerator

...
# some other imports 
...

# adapts all prompts to a given language, here german
for p in testset_prompts:
    p.adapt("german", translator_llm)

# run the test set generation
generator = TestsetGenerator(
        docstore=docstore,
        generator_llm=generator_llm,
        critic_llm=critic_llm,
        embeddings=embeddings
    )

dataset = generator.generate(
        test_size=TEST_SIZE,
        distributions=DISTRIBUTION
)

Without this fix, this would only work the first time. The first time, when the prompts are translated the "adapt" method changes the prompt instances (that is, in place). Therefore, the generate below uses the translated prompts. However once the prompts are in the cache, running adapt on all prompts doesn't change them "in place" and then generate will access the original prompts completely in English. This PR makes this approach to work also when the translated prompts are already cached.

I also added the testset_prompts variable for convenience.