The existing model of forwarding to the quiche::MemSliceImpl constructor is an abstraction violation and makes it hard to swap out the underlying MemSlice platform implementation. As such, we've recently added a new constructor to QuicheMemSlice (and the Impl API) that allows for an arbitrary custom releasor.

This PR uses the new constructor, guarded behind the runtime flag envoy.reloadable_features.quiche_use_mem_slice_releasor_api (disabled by default).

Since we're storing a unique_ptr in the capture list (allowable since C++17), we can't store this in a std::function which requires copyability. As such, we use absl::AnyInvocable.

(I've marked this as medium risk since it interacts with Envoy's memory management model for QUIC streams and so deserves some scrutiny.)

Commit Message: Use QuicheMemSlice constructor with releasor.
Additional Description:
Risk Level: Medium
Testing: This is a refactor, so it should be covered by existing tests (test/common/buffer:buffer_test, test/common/quic:envoy_quic_{client,server}_stream_test)
Docs Changes: n/a
Release Notes: n/a
Platform Specific Features: n/a

Request variables

Key	Value
ref	d3536b9a62a6c1b322b7882b1a126f4708f1fe8f
sha	44af392
pr	31167
base-sha	00fe753
actor	@steveWang
message	Use QuicheMemSlice constructor with releasor....
started	1702316828.335601
target-branch	main
trusted	false

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