Platform channels, eliminate objc copies

[gaaclarke]

issue: flutter/flutter#81559
depends on: #25867

locally tested
before:

flutter: BasicMessageChannel/StandardMessageCodec/Flutter->Host/Small: 153.3 µs
BasicMessageChannel/StandardMessageCodec/Flutter->Host/Large: 1246.7 µs
BasicMessageChannel/BinaryCodec/Flutter->Host/Large: 143.2 µs
BasicMessageChannel/BinaryCodec/Flutter->Host/1MB: 1218.6 µs

after:

flutter: BasicMessageChannel/StandardMessageCodec/Flutter->Host/Small: 152.7 µs
BasicMessageChannel/StandardMessageCodec/Flutter->Host/Large: 1192.5 µs
BasicMessageChannel/BinaryCodec/Flutter->Host/Large: 150.2 µs
BasicMessageChannel/BinaryCodec/Flutter->Host/1MB: 761.2 µs

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.
• The reviewer has submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

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Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[gaaclarke]

@chinmaygarde This is the last PR in my series of PR's to speed up platform channels iOS by eliminating copies. It was all building up to this moment.

[chinmaygarde]

Apologies for the delay. This looks great. Thanks.

[gaaclarke]

This finally landed and has official results from our tests. It reduced the cost of sending 1mb over binary platform channels 48% on iOS: https://flutter-flutter-perf.skia.org/e/?begin=1620764044&end=1621044607&queries=sub_result%3Dplatform_channel_basic_binary_2host_1MB%26test%3Dmac_ios_platform_channels_benchmarks_ios&requestType=0