refactor: cleanup template functions

[buchdag]

This PR cleans up the template functions.

json / parseJson / sha1 have been made alias of Sprig functions that behave identically:

docker-gen	sprig
json	mustToJson
parseJson	mustFromJson
sha1	sha1sum

trimPrefix / trimSuffix have been removed because Sprig already have functions with the same name that do the same thing.

Sprig function that have the same name as existing docker-gen functions and where thus masked have been made available with the sprig prefix:

• sprigCoalesce
• sprigContains
• sprigDir
• sprigReplace
• sprigSplit
• sprigSplitn