Add key_manager ffi in tiflash_compute mode

[guo-shaoge]

Enhancement

In tiflash_compute mode, no need to run proxy, but we still need key_manager to do encryption for local cache.

Need to implement key_manager using rust ffi. So we can use it without starting proxy.

[CalvinNeo]

Enhancement

In tiflash_compute mode, no need to run proxy, but we still need key_manager to do encryption for local cache.

Need to implement key_manager using rust ffi. So we can use it without starting proxy.

Is the only-decryption similar to what you need?

[guo-shaoge]

@CalvinNeo What is only-decryption, do we have any doc?