[Feat] Allow specification of chat template

[kalantar]

Per https://docs.vllm.ai/en/latest/serving/openai_compatible_server.html#chat-template:

In order for the language model to support chat protocol, vLLM requires the model to include a chat template in its tokenizer configuration. The chat template is a Jinja2 template that specifies how are roles, messages, and other chat-specific tokens are encoded in the input.

The default template does not work for all models. For example the facebook/opt-125m model requires a different template.

This PR modifies the helm chart to allow a modelSpec to specify a chat template. For example, for facebook/opt-125m:

    chatTemplate: |-
      {% for message in messages %}\n{% if message['role'] == 'user' %}\n{{ 'Question:\n' + message['content'] + '\n\n' }}{% elif message['role'] == 'system' %}\n{{ 'System:\n' + message['content'] + '\n\n' }}{% elif message['role'] == 'assistant' %}{{ 'Answer:\n'  + message['content'] + '\n\n' }}{% endif %}\n{% if loop.last and add_generation_prompt %}\n{{ 'Answer:\n' }}{% endif %}{% endfor %}

The template is saved in a ConfigMap and mounted as a volume.

FIX #xxxx (link existing issues this PR will resolve)

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