feat($animate): add support for `customFilter`

This commit adds a new `customFilter()` function on `$animateProvider` (similar
to `classNameFilter()`), which can be used to filter animations (i.e. decide
whether they are allowed or not), based on the return value of a custom filter
function.
This allows to easily create arbitrarily complex rules for filtering animations,
such as allowing specific events only, or enabling animations on specific
subtrees of the DOM, etc.

Fixes #14891