LLM: Add support for Ernie chat_completions

[sxk10812139]

The current implementation lacks support for Ernie chat_completions, so I've added code for this. The committed changes include basic stream calls, function calls, and other capabilities.

The invocation of Ernie is based on the official documentation at https://cloud.baidu.com/doc/WENXINWORKSHOP/s/jlil56u11.

For documentation on this content in the Python version of Langchain, please refer to https://python.langchain.com/docs/integrations/chat/ernie.

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.

[sxk10812139]

Can this PR be merged? Our project will use this feature. Please help confirm. Thank you. @tmc

[tmc]

LGTM