Add `difference` filter to Jinja context

Ansible has a few useful filters in their Jinja context that are not part of core Jinja. It’d be great to expose some of them to macro developers dbt.

For example, difference is a helpful filter to get the difference between two lists.

    @contextfilter
    def difference(context, first, second):
        second = set(second)
        return [item for item in first if item not in second]

Used like so:

{%- set columns = columns | difference(except_columns) -%}

Since this would be run in Python, there should be no cross-database issues.

Related to #480 maybe although I don’t think it’s necessary to do from an end-user perspective, as long as its easy to enough to PR new functions like this.

[joellabes]

Huh! Turns out I created a dupe of this in https://github.com/dbt-labs/dbt-core/issues/4571 - going to close that one and reopen this one. cc @gshank who triaged my one and might be able to provide some pointers of where to add things.

For the record, I was also jealous of Ansible’s casting datatypes in addition to the set theory functions

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