Add downsampling spec for data stream lifecycle

[andreidan]

This adds the specification for downsampling as part of
the data stream lifecycle.

The downsampling configuration is nullable (just like
data_retention) and itself can contain a nullable list
of downsampling rounds.

Each round consists of an after duration/timevalue field
and the downsample configuration (the fixed_interval)

[andreidan]

@elastic/clients-team this build is failing with

build
Input required and not supplied: token

Can someone please advise?

[JoshMock]

Not sure what that build error is about, but fixing this typo will rerun the job anyway. 🙂

[andreidan]

Thanks @JoshMock Seems like the build is happy now

[gmarouli]

LGTM, as discussed offline, we agreed that we should allow the client to nullify the rounds. The user can use the downsampling?: DataStreamLifecycleDownsampling to express a nullification during template composition. Apart from that 🚀

[andreidan]

Superseeded by #2259