This adds a new channel argument `GRPC_ARG_DSCP` which allows users to
create classified gRPC streams with a
Differentiated Services Code Point (DSCP) marking on the IP frames.
The channel argument is handled on both clients and servers, but
currently only on posix based systems.
Fixes#17225
**Background**:
In addition to what is already described is #17225, when gRPC is used in
telco systems there is often a need to classify streams of importance.
There can be multiple hops between two endpoints (e.g. between 2 telecom
operators) and some streams that are more important than others (e.g.
emergency call related or similar). By marking the IP packets using DSCP
the aware routers can make a sound decision of the prioritization.
This PR propose to use DSCP as the configuration value since its common
for both IPv4/IPv6, an alternative would be to use a config name that
includes TOS and Traffic Class.
There might be more needed regarding documentation and end2end testing,
but there I need some advice.
**References**
https://datatracker.ietf.org/doc/html/rfc2474https://www.iana.org/assignments/dscp-registry/dscp-registry.xhtml
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