Add a collector for Rekor

[rgreinho]

Adds a new collector for Rekor.

Fixes guacsec/guac#68

[Hayden-IO]

I'll take a look early next week to give some more detailed comments around what I think is necessary and what's not for collecting entries from Rekor.

Also we're looking to add an API to fetch entries by a range of indices, and we're looking at adding some support for pushing entries to a pub/sub queue (both would be coming at some point, no ETA)

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity (60 days of inactivity). It will be closed in 30 days if no further activity occurs. Thank you for your contribution!

[stale[bot]]

This pull request has been automatically closed because there has been no activity for 90 days. Please feel free to reopen it (or open a new one) if the proposed change is still appropriate. Thank you for your contribution!