Add get_ref/get_mut to JSON Writer

[cetra3]

Which issue does this PR close?

None

Rationale for this change

I need access to the writer so that I can flush an external buffer when bytes are written.

What changes are included in this PR?

A couple of methods to the JSON writer. These methods already exist on other writers

Are these changes tested?

N/A

Are there any user-facing changes?

Yes, a couple extra methods on the JSON writer.

[adriangb]

@alamb this is a minor simple change, as Peter said it's already on the IPC writer:

arrow-rs/arrow-ipc/src/writer.rs

Lines 1116 to 1126 in 42bb5b4

	/// Gets a reference to the underlying writer.
	pub fn get_ref(&self) -> &W {
	&self.writer
	}
	/// Gets a mutable reference to the underlying writer.
	///
	/// It is inadvisable to directly write to the underlying writer.
	pub fn get_mut(&mut self) -> &mut W {
	&mut self.writer
	}

Could we get this merged?

[alamb]

Thank you @cetra3 and @adriangb
I think the API makes a lot of sense, but I think we should clarify the comment on get_mut