Expose credential_count in feeds API and add min/max credential count filters

[drona-gyawali]

Following up on #1098 , I want to purpose a enhancement on api by adding two query parameters max_credential_count and min_credential_count so researchers can see how many distinct credentials were used against each IOC

I'd love to be work on this @regulartim , if it aligns with the current direction.

[regulartim]

Hey @drona-gyawali ! Sure, I'll assign you!