Parse IdentityEntries into MonitoredIdentities

[linus-sun]

Summary

Per specification here, this PR implements the following:

• JSON tagging for the IdentityEntry struct (part of structured JSON output for use in custom notification handling)
• the MonitoredIdentity struct for notification handling
• the NotificationPlatform interface to be extended by custom notification platforms
• the CreateMonitoredIdentities function to parse a list of IdentityEntries into a list of MonitoredIdentities, for use in creating structured output of found identities from a run of rekor-monitor.

Release Note

NONE

Documentation

none needed for this PR, changes to documentation will be made in future respective PRs which implement API changes

[codecov]

Codecov Report

Attention: Patch coverage is 59.57447% with 19 lines in your changes missing coverage. Please review.

Project coverage is 56.19%. Comparing base (d271ec7) to head (432d364).
Report is 103 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/rekor/identity.go	59.57%	16 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #458      +/-   ##
==========================================
- Coverage   64.02%   56.19%   -7.84%     
==========================================
  Files           4        9       +5     
  Lines         303      694     +391     
==========================================
+ Hits          194      390     +196     
- Misses         78      239     +161     
- Partials       31       65      +34     

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[linus-sun]

I'm also open to a refactoring of IdentityEntry into pkg/identity, which would move this code into there as well- more broadly, what should the specifications be determining if code should go in pkg/rekor/identity vs pkg/identity?