Add preload cache clear

[fauh45]

Add a function to clear out the preload cache for the use case where calls to useSWR are dynamic, thus the data are not loaded on to the normal cache, and just being left there on the cache.

Background use case: #4193

• Added clearPreloadCache to clear preload cache
• Added test to see if clearPreloadCache would work

[codesandbox-ci]

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