Add support for more kinds of recovery targets

[gpothier]

It is not currently possible to specify any type of recoveryTarget other than targetTime in the cluster bootstrap:

    {{- with .Values.recovery.pitrTarget.time }}
    recoveryTarget:
      targetTime: {{ . }}
    {{- end }}

What is your feeling about breaking changes*? I would suggest this:

    {{- with .Values.recovery.recoveryTarget }}
    recoveryTarget:
      {{- toYaml . | nindent 6 }}
    {{- end }}

Which means that:

• To get the same result as now, one would need to specify recovery.recoveryTarget.targetTime instead of recovery.pitrTarget.time
• It would be possible to specify other kinds of supported targets, eg. recovery.recoveryTarget.targetImmediate
• The nomenclature between the chart and the "native" Cluster would be the same, instead of introducing a new term pitrTarget

*Actually, it is actually simple to maintain backward compatibility if needed: instead of replacing the first snippet by the second one, we could keep both.

What do you think? Would you take a PR for this change?

[itay-grudev]

I originally wanted to keep it simple, but I'm accepting PRs for this.