Allows cluster filtering in account subject mapping

[jnmoyne]

• Branch rebased on top of current main (git pull --rebase origin main)
• Changes squashed to a single commit (described here)
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Changes proposed in this pull request:

Allows (exposes) the existing cluster filtering option of account level subject mapping, thereby allowing you to define different mapping destination per cluster (e.g. to insert the cluster name as a subject token).

Example:

mappings {
	"foo":[
		{destination:"foo-east", cluster: "east", weight:100},
		{destination:"foo-west", cluster: "west", weight:100},
	]
}

[derekcollison]

LGTM