hexdecode function addition

[akingscote]

Added functionality for hex decode.

Im a bit of a novice so please let me know what ive missed

Please see my related issue here: #26163

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Update: i have changed the output to return a base64 encoded representation rather than the raw binary as per @apparentlymart comment on my issue.

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[codecov]

Codecov Report

Merging #26214 into master will increase coverage by 0.00%.
The diff coverage is 77.77%.

Impacted Files	Coverage Δ
lang/funcs/encoding.go	73.80% <75.00%> (+0.28%)	⬆️
lang/functions.go	92.68% <100.00%> (+0.05%)	⬆️
dag/marshal.go	54.44% <0.00%> (+1.11%)	⬆️

[pawelprazak]

this would be convenient to workaround some integration issues Azure KeyVault