feat: getDiskCacheSize and getMemoryCacheSize

[phalera]

Describe the bug
Does it have a static function?
Maybe user needs to know before clear cache( such as 15MB or 1234KB)

To Reproduce
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Expected behavior
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Screenshots
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Dependency versions

• React Native version:0.65.1
• React version:
• React Native Fast Image version:Lastest

[DylanVann]

This does not currently exist. IMO not needed since the cache is first in first out, you don't need to manually manage it.

There could possibly be two functions:

• getDiskCacheSize
• getMemoryCacheSize