Alter Layer handling when requesting `extra_info`

The current behaviour when requesting `extra_info` is to have one feature per line segment in the output line that has the requested `extra_info`-values as attributes.

For longer routes, this results in a lot of features.
The intuitive assumption, however, would be that all features where the `extra_info` attribute combination doesn't change are dissolved into one feature, especially when only requesting one `extra_info`.

When shorter routes are requested with multiple parameters in `extra_info`, there are a lot of attribute combinations, so dissolving wouldn't change much.

Additionally, dissolving can be done using QGIS builtin processing tools.

There exist a few different approaches to address this issue:
1. [Current] do not dissolve and leave the task to the user
1. Dissolve only when one `extra_info` is requested. This could even be forced by setting `allowMultiple=False` on the processing parameter.
1. Return one layer per requested `extra_info` and dissolve that
1. Dissolve on every change of `extra_info` attribute combination

For every approach, dissolving could be made optional via another parameter.

There might be use cases, where the non-dissolved output is the wanted output, so every dissolve would have to be undone.