feature: add filter to cache methods

[bitfactory-robin-martijn]

Related:

• add page slug to transient name #2604

Issue

The transient keys used for caching are hard to determine, making it hard to work with them as well.

Solution

This PR adds a filter as proposed in the issue, which allows developers to modify the transient key. It gives a maximum of 4 variables to work with:

/*
 * @param string $trans_key The transient key.
 * @param string $key The cache key.
 * @param string $group The cache group.
 * @param string $cache_mode The cache mode.
*/

Impact

As this is a new filter, it does not affect existing code.

Usage Changes

I added the parameters in a docblock as well. Although I am not sure if I did that the right way, so assistance would be appreciated here.

Considerations

This is the only way I see to help developers such as those in the original issue, without modifying existing transient keys.

Testing

I added a test to see if the filter works properly, which passes.

[Levdbas]

Thank you @expedition-robin-martijn , this looks good to me. Thank you for taking the time for adding the filter docblocks!

[nlemoine]

All good, thank you @expedition-robin-martijn !