Override `highWaterMark` didn't work

Start at the awslambda.streamifyResponse entry point and trace how its highWaterMark option and handler symbol are consumed. Reproduce the two examples and verify that a configured value of 512 delivers the early writes before the delayed writes.

The following code will send the data BEFORE 6s:

const util = require('node:util');
const HANDLER_HIGHWATERMARK = Symbol.for(
  'aws.lambda.runtime.handler.streaming.highWaterMark',
);

const handler = async (event, responseStream, context) => {
        responseStream.setContentType('text/html; charset=utf-8');

        responseStream.write("Thinking...");
        responseStream.write("a".repeat(64 * 1024));

        setTimeout(function() {
            responseStream.write("b".repeat(64 * 1024));
        }, 3000);

        setTimeout(function() {
            responseStream.write("After 6s");
            responseStream.end();
        }, 6000);
    };

handler[HANDLER_HIGHWATERMARK] = 512;

exports.handler = awslambda.streamifyResponse(handler, {
        highWaterMark: 512
    }
);

The following code will only send data AFTER 6s.

const util = require('node:util');
const HANDLER_HIGHWATERMARK = Symbol.for(
  'aws.lambda.runtime.handler.streaming.highWaterMark',
);

const handler = async (event, responseStream, context) => {
        responseStream.setContentType('text/html; charset=utf-8');

        responseStream.write("Thinking...");
        responseStream.write("a".repeat(1 * 1024));

        setTimeout(function() {
            responseStream.write("b".repeat(1 * 1024));
        }, 3000);

        setTimeout(function() {
            responseStream.write("After 6s");
            responseStream.end();
        }, 6000);
    };

handler[HANDLER_HIGHWATERMARK] = 512;

exports.handler = awslambda.streamifyResponse(handler, {
        highWaterMark: 512
    }
);

From what I tested, the default highWaterMark is 64Kb, but there is no way to override it to be lower, I tried two different ways to do it.

This causes weird issues where I can't send some tiny text and then wait 6s to send more data, it will be sent as a single chunk only after 6s.

This issue was initially discovered at https://github.com/H4ad/serverless-adapter/issues/166