<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
This is part of
https://github.com/open-telemetry/opentelemetry-collector/issues/13106
It introduces a new `SetMapping` method, which allows setting a
location's mapping based on a specific dictionary.
I named the method `SetMapping` and not `PutMapping`, as it overrides
the `MappingIndex` value, it doesn't append to a slice.
Contrarily to Locations or Attributes, as location has a single mapping.
So we don't need a `FromMappingIndices` method, we can use
`MappingSlice.At()` directly.
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Co-authored-by: Tim Rühsen <tim.ruehsen@gmx.de>
Damien Mathieu