templates: make "join" work with non-string slices and map values

[thaJeztah]

• relates to https://github.com/docker/cli/pull/2733

Add a custom join function that allows for non-string slices to be joined, following the same rules as "fmt.Sprint", it will use the fmt.Stringer interface if implemented, or "error" if the type has an "Error()".

For maps, it joins the map-values, for example:

docker image inspect --format '{{join .Config.Labels ", "}}' ubuntu
24.04, ubuntu

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[codecov-commenter]

Codecov Report

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