Add cache prefetch opt out filter

[daledupreez]

Related to:

• Implement cache prefetch for payment method configuration #4637

Changes proposed in this Pull Request:

This PR adds a filter to allow the default cache prefetch window for supported keys to be overridden by merchants, as suggested by @diegocurbelo in this comment on #4637.

The new filter is wc_stripe_database_cache_prefetch_window, and takes two arguments:

• The incoming prefetch window, which should be an integer
• The cache key

The filter should return a negative integer or 0 to disable cache prefetching for a specific key. If it returns a positive integer, that will be used as the cache prefetch window. If any other value is returned, we fall back on the default prefetch window defined in WC_Stripe_Database_Cache_Prefetch::PREFETCH_CONFIG.

The filter is only triggered for supported cache keys that have an entry in WC_Stripe_Database_Cache_Prefetch::PREFETCH_CONFIG.

Testing instructions

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• Covered with tests (or have a good reason not to test in description ☝️)
• [N/A] Tested on mobile (or does not apply)

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