add extendable chunk "discovery" option

[martonilles]

During extraction when an unknown chunk occurs it would be great to show hints & insights on the unknown chunk so the user can easier analyze the un-identify chunks in a blob.

Currently we do entropy calculation, but there are other possible options to provide hints on the unknown chunks, eg:

• identify if the complete chunk is just padding (only 0x00 or 0xff bytes) and flag the unknown chunk as padding
• identify other known (but currently not extracted) patterns

It would be useful if the user could also add other "simple" discovery options that can report hints as well

[martonilles]

Similar to #263

[qkaiser]

Being worked on in branch https://github.com/onekey-sec/unblob/tree/padding-auto-id