This PR removes running the callbacks with the Executor in the API. Although, it makes sense to run the certain callbacks with the Executor, especially for blocking I/O so that we don't block the Envoy thread, having the API that forces the use of Executor is bad because we will always need to copy the ByteBuffer before running it with the Executor or else we may run into accessing a free-ed ByteBuffer. This change is essentially making the use of the Executor for the callbacks a user choice. Both the EnvoyHTTPCallbacks and EnvoyHTTPFilter interfaces have been updated with the comments related to using ByteBuffer data. The change also makes the Java/Kotlin API consistent with the other language APIs.

The PR is also a follow-up to #32715 to reduce the number of copies, especially for item 5 in the after section. With this change, there is only one copy from Envoy::Buffer::Instance to envoy_data.

For Cronvoy, there is no substantial change because the CronvoyUrlRequest#onData already makes a copy of the ByteBuffer passed from the callback before passing it into a separate thread. The only change is to make a copy earlier prior to spawning a thread.

Risk Level: high
Testing: CI
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: mobile

Request variables

Key	Value
ref	f020c18516fa28906f1b59c62e90a2be3e3a5fe7
sha	18b9a44
pr	32776
base-sha	a3406aa
actor	@fredyw
message	mobile: Remove Executor from Java & Kotlin API...
started	1709849147.339477
target-branch	main
trusted	false

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Envoy version (as used in this CI run)