Add vLLM support to DocSum Helm chart

[eero-t]

Description

This continues Helm vLLM support added in #610, by adding vLLM support to DocSum Helm chart.

(Similarly to how it's already done for ChatQnA app + Agent component, there are tgi.enabled & vllm.enabled flags for selecting which LLM will be used.)

Type of change

• New feature (non-breaking change which adds new functionality)

Dependencies

opea/llm-docsum-vllm:latest image is currently missing from CI & DockerHub registries:
opea-project/GenAIComps#961

(Although corresponding opea/llm-docsum-tgi:latest image for TGI, and opea/llm-vllm:latest vLLM text-generation images already exist.)

Tests

Manual testing with opea/llm-docsum-vllm:latest image built locally.

[eero-t]

Setting as draft because the required image is still missing from DockerHub, and this needs retesting after currently pending DocSum changes for Comps & Examples repos have completed.

[eero-t]

While CI "docsum, gaudi, ci-gaudi-vllm-values" test fails as expected, due to OPEA missing llm-docsum-vllm image...

There seems to be a bug in component unrelated to this PR, as also run "llm-uservice, xeon, ci-faqgen-values, common" CI test fails to a package missing from image:

[pod/llm-uservice20241218190439-5b9b7b79fd-r65l9/llm-uservice20241218190439]
...
   File "/home/user/comps/llms/faq-generation/tgi/langchain/llm.py", line 77, in stream_generator
     from langserve.serialization import WellKnownLCSerializer
   File "/home/user/.local/lib/python3.11/site-packages/langserve/__init__.py", line 8, in <module>
     from langserve.client import RemoteRunnable
   File "/home/user/.local/lib/python3.11/site-packages/langserve/client.py", line 24, in <module>
     from httpx._types import AuthTypes, CertTypes, CookieTypes, HeaderTypes, VerifyTypes
 ImportError: cannot import name 'VerifyTypes' from 'httpx._types' (/home/user/.local/lib/python3.11/site-packages/httpx/_types.py)

=> requirements.txt for llm-faqgen-tgi:latest image generation is not up to date in Comps repo?

@lianhao?