Use ellmer as backend for chat addin

[calderonsamuel]

Related issue

• Closes [FR]: Use {ellmer} as "engine" for chat functionality #247
• Closes [Bug]: Read R help pages not working for all providers #242
• Closes [FR]: Add streaming support for other models #221

Description of changes

• Migrated the logic for performing API requests to use {ellmer} as backend. #247
• Added streaming support for all the currently supported providers (previously only available for OpenAI and Ollama). #221
• The "Read docs" functionality now works for every Provider. #242
• [INTERNAL] Added tests for the chat() wrapper on every migrated provider (missing azure_openai). Also, updated several tests and snapshots.

For contributors

• I have added the relevant changes to the NEWS.md file
• I have added relevant tests or documentation with my changes

For reviewers

• Changes meet the acceptance criteria of the related issue
• The contribution follows style conventions and code of conduct
• Branch passes automated testing
• I have incremented the package version in the DESCRIPTION file before merging

[calderonsamuel]

@JamesHWade let me know if there is anything missing on the azure_openai implementation. We can treat documentation as a separate issue. Might be a great time to start a "release issue" for v1.0.0

[JamesHWade]

Yes. Let's create the v1.0.0 release issue!!! I still need to fix azure for some edge cases before we push to CRAN but it should work for most use cases.