Fix performance degradation in sinks due to slow writer in hot path

[pablosichert]

After integrating encoding::Encoder with sinks, performance dropped by roughly 20% when using JSON encoding: (e.g. for socket #10684 (comment), or for loki #12558 (comment)).

As evident by #10684 (comment) this can be explained by poor writer performance in the encoding hot path. Related issues: #11341 and tokio-rs/bytes#531.

[pablosichert]

Related to this, some improvements could be made by reducing the allocations in encode_input

vector/src/sinks/util/encoding/mod.rs

Lines 112 to 123 in 7cc3a80

	for mut event in events {
	self.0.transform(&mut event);
	let mut bytes = BytesMut::new();
	encoder
	.encode(event, &mut bytes)
	.map_err(|error| io::Error::new(io::ErrorKind::InvalidData, error))?;
	writer.write_all(&bytes)?;
	bytes_written += bytes.len();
	}
	let mut event = last;
	self.0.transform(&mut event);
	let mut bytes = BytesMut::new();

as mentioned by @tobz in #13518 (comment)