TimeRange: Parse duration with d/M/y unit

[AgnesToulet]

What this PR does / why we need it:
This PR updates the parse function to use our own ParseDuration function instead of the Golang one. The Golang one doesn't handle unit like d, M or y that are valid units in Grafana frontend when setting a panel relative time range:

Which issue(s) this PR fixes:

https://github.com/grafana/support-escalations/issues/14375

Special notes for your reviewer:

• I tested it for the escalation use case and it's working but I wonder if there are any other use cases where this could cause an unexpected behavior.
• This would require a more complex fix if we want to handle daylight saving time/leap years/proper month duration... But I don't think it's worth it as this can already work by using the syntax now-1M. Maybe we should update the doc to recommend using the second syntax instead of the first one?

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[matyax]

This seems to be a shared component with Loki. If you don't mind, I added @svennergr as reviewer to make sure this doesn't cause any unexpected regression.