Introduces UdpPacketWriter Interface that can be used to perform writes in Batched/PassThrough modes by using QuicGsoBatchWriter implementation from QUICHE extension.
**Additional Description:**
UDP GSO (Generic Segmentation Offload) was introduced in Linux at version 4.18. It allows batch-writing of multiple messages into a single payload and sending these messages along as a batch in a single sendmsg syscall.
Currently, Envoy performs the sending of messages using simple sendmsg implementation in pass-through mode, i.e. no support for batch writing. With this change, UdpListener can use UdpPacketWriter interface as a DefaultWriter or a GsoBatchWriter to perform pass-through or batched writes respectively.
Detailed description of the changes can be found in the design document, [here](https://docs.google.com/document/d/16ePbgkfrzQ6v-cOVMSnKDja3dUdZvX-mxT9jw29rx4g/edit?usp=sharing).
**Risk Level:** Low, not in use
**Testing:**
- Added udp_listener_impl_batched_writes_test, to verify that multiple packets of varying sizes are batched/flushed as per gso specifications while using UdpGsoBatchWriter.
- Modified existing tests, to verify that UdpDefaultWriter performs writes in pass-through mode.
- Ran all tests. All 677 tests passed successfully.
```
**Docs Changes:** None
**Release Notes:** None
**Fixes:** #11925
Signed-off-by: Yugant <yugant@google.com>
Mirrored from https://github.com/envoyproxy/envoy @ 933e267db60961e76549ae6e65ac4872cb78db4d
data-plane-api(CircleCI)
66443ebd3b