Jsonnet: add distributor_node_affinity_matchers setting support

[pracucci]

What this PR does

As part of my on-going work to deploy distributors in multi-AZ, I need an easy way to configure node affinity rules. In this PR I'm proposing to add distributor_node_affinity_matchers so that different jsonnet files can append to it, and then the matchers are materialised by newDistributorDeployment().

In this PR I'm doing it only for distributors, but if experiment will be successful we can do it for all components.

The test I've added can be compared against test-defaults-generated.yaml:

$ diff operations/mimir-tests/test-defaults-generated.yaml operations/mimir-tests/test-node-selector-and-affinity-generated.yaml

434a435,448
>       affinity:
>         nodeAffinity:
>           requiredDuringSchedulingIgnoredDuringExecution:
>             nodeSelectorTerms:
>             - matchExpressions:
>               - key: topology.kubernetes.io/region
>                 operator: In
>                 values:
>                 - us-east-1
>               - key: topology.kubernetes.io/zone
>                 operator: In
>                 values:
>                 - us-east-1a
>                 - us-east-1b
490a505,506
>       nodeSelector:
>         workload: mimir
585a602,603
>       nodeSelector:
>         workload: mimir

... many more repetitions of the nodeSelector ...

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[dimitarvdimitrov]

LGTM, thank you