Refactor `Context::with_context` into `Context::to_borrowed`

[analogrelay]

We have Context::with_context<'b>(other: &Context<'a>) -> Context<'b> that allows you to "reborrow" a Context<'a> into some smaller lifetime Context<'b>. However, in #2577 we identified that it feels cleaner to call this as a method instead of a constructor. This issue tracks refactoring it into a method: Context<'a>::to_borrowed<'b>(&self) -> Context<'b>

[heaths]

@analogrelay and I talked about this offline and it's definitely a lot cleaner. I think both ways are fine. @JeffreyRichter, what do you think? It's the same code, but when chaining contexts during construction, the with_context() follows our pattern of construction. to_borrowed() is more idiomatic when passing to other functions, though.

[analogrelay]

Just to illustrate the usage difference:

let context = Context::new().with_value(SomeValue);

// Current approach:
client.some_method(Some(ClientMethodOptions { context: Context::with_context(context) }));

// Proposed approach
client.some_method(Some(ClientMethodOptions { context: context.to_borrowed() }));