Provide built-in web fetch tool

[hadley]

From my experience, a lot of sites are blocking the built-in web fetch tools so it would be nice to provide an alternative.

[gadenbuie]

An option from btw: https://posit-dev.github.io/btw/reference/btw_tool_web_read_url.html

library(ellmer)

chat <- chat_openai()
chat$register_tools(btw::btw_tools("web"))

# or
chat <- btw::btw_client(client = "openai", tools = "web")

[hadley]

I forgot that converting to markdown would really need to be part of it. So I think ellmer doesn't need to do this, but when I add support for built-in web fetch, I'll also document the btw solution.

[hadley]

Will note in the news bullet and in the blog post.