ping/_compositions/rust: Add v0.49.0 and update features of master

[mxinden]

With libp2p/rust-libp2p#3001 master is now v0.50.0 AND removes the dns-async-std etc. features.

This results in test failures, see e.g. libp2p/rust-libp2p#3047.

We need to do the upgrade of libp2p/test-plans similar to #41.

Most prominently, we need to update the versions below:

test-plans/ping/_compositions/rust.toml

Lines 1 to 9 in c71602e

	[master]
	CargoFeatures = 'libp2pmaster'
	[custom]
	CargoFeatures = 'libp2pmaster'
	[[groups]]
	Id = '0.48.0'
	CargoFeatures = 'libp2pv0480'

and update the `Cargo.toml.

//CC the rust-libp2p team @thomaseizinger @jxs @elenaf9 for visibility and in case one of you wants to pick this up.

[thomaseizinger]

Working on this now.