quic: Reduce QuicheMemSlice memory footprint from ~968 B to ~120 B

[filipcacky]

Commit Message: quic: Reduce QuicheMemSlice memory footprint from ~968 B to ~120 B
Additional Description:

Change QuicheMemSlice backing storage from Buffer::OwnedImpl to Buffer::SliceDataImpl. This reduces the memory footprint for one slice from ~968 B from the buffer to ~120 B and removes the OwnedImpl ctor from the hot path.

Risk Level: Low
Testing: Added tests for extractImmutableFrontSlice
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

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Caused by: #46724 was opened by filipcacky.

see: more, trace.

[filipcacky]

We noticed QUIC send buffers using more memory than necessary on connections over congested/lossy networks, where slices may need to stay in the buffer for longer periods of time.

In our case some internal filters fragment buffers and end up with many small slices. I don't think this is a significant issue for stock Envoy. In any case this reduces the per-slice overhead and removes the OwnedImpl ctor from the write path.

Also removed the QuicheMemSliceImpl, which isn't required by quiche anymore.