Add `timestamp` attribute to LlmChatCompletionMessage and LlmChatCompletionSummary events

[hannahramadan]

LlmChatCompletionMessage and LlmChatCompletionSummary events should now include a timestamp attribute.

The timestamp value should not be the timestamp of when the custom event was created but rather the timestamp of when the LLM request was sent.

spec

[Newrelic-Boomi]

https://new-relic.atlassian.net/browse/NR-522008