Updating Alibi Detect Custom Metrics Server to use Dill instead of Pickle

[axsaucedo]

What this PR does / why we need it:

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[seldondev]

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[axsaucedo]

/test integration

[axsaucedo]

/test release

[seldondev]

@axsaucedo: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
release-build-push	8eb020c	link	/test release

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