[1.6] Add Fields type alias to log package

[neersighted]

Partial backport of #8143, to make consuming github.com/containerd/containerd/log easier for those on the 1.6 LTS branch. Given the original commit is large and invasive, it seemed better to go for the direct result, rather than make a dirty cherry-pick (and miss many usages in this branch).

Motivated in large part by moby/moby#45799, but likely useful to other projects as well.

The original PR body is reproduced below:

---

When logging with custom fields, like:

log.G(ctx).WithFields(logrus.Fields{ ... })...

you're forced to do two imports

"github.com/containerd/containerd/log"
"github.com/sirupsen/logrus"

This PR adds a type alias to log package, so we end up with just one import:

"github.com/containerd/containerd/log"
 
log.G(ctx).WithFields(log.Fields{})

Which often reduces the number of imports and makes log package a bit nicer.

[k8s-ci-robot]

Hi @neersighted. Thanks for your PR.

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[neersighted]

/cc @thaJeztah @mxpv @fuweid @dcantah

[dcantah]

Makes sense to me for the partial cp

[thaJeztah]

LGTM

do we want the other aliases, such as the log-levels as well (in a follow-up)?

containerd/log/context.go

Line 44 in 9b4ed8a

Level = logrus.Level

[fuweid]

I think it's ok to cherry-pick all content from the commit if there is no conflict. It can be easy to track.

[neersighted]

There are extensive conflicts (> 50%) and some usages will be missed even if I cherry-pick all s/logrus/log commits from 1.7.

I am happy to also do the other aliases if there is interest, but there seems to be less in the way of potential consumers.

[fuweid]

@neersighted thanks for the update. LGTM