[Feature] Support custom layouts to snap yaml

[VerteDinde]

Layouts are a feature for strictly confined snaps that modify the execution environment.

With layouts, you can make elements in $SNAP, $SNAP_DATA, $SNAP_COMMON accessible from locations such as /usr, /var and /etc. This helps when using pre-compiled binaries and libraries that expect to find files and directories outside of locations referenced by $SNAP or $SNAP_DATA.

Given our existing support for custom plugs and slots, we can use a similar framework to optionally add layouts if they are provided by the user. Layouts do not need to be included by default.

Layouts are only supported on core18 and higher.

[welcome]

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