[metricstarttimeprocessor] Factor start time cache out into generic cache

[dashpole]

Component(s)

processor/metricstarttime

Is your feature request related to a problem? Please describe.

The metric start time processor will need to cache start timestamps and values for timeseries it processes for a variety of strategies.

The current cache is part of the true_reset_point strategy: https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/processor/metricstarttimeprocessor/internal/truereset/start_time_cache.go

Describe the solution you'd like

We should move that to its own internal directory, and the type it stores generic so that it can be re-used by other strategies.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

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