CertStore: Implement a friendlier subscribe method

[Stebalien]

I ended up not needing this but I wanted to get some feedback on it as it's significantly friendlier than the current SubscribeForNewCerts method. Basically, this one doesn't force the subscriber to implement resubscribe/catchup logic manually.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 47 lines in your changes missing coverage. Please review.

Project coverage is 80.95%. Comparing base (69e5251) to head (a6a90e2).
Report is 183 commits behind head on main.

Files with missing lines	Patch %	Lines
certstore/certstore.go	0.00%	47 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
- Coverage   83.08%   80.95%   -2.14%     
==========================================
  Files          15       15              
  Lines        1780     1827      +47     
==========================================
  Hits         1479     1479              
- Misses        175      222      +47     
  Partials      126      126              

Files with missing lines	Coverage Δ
certstore/certstore.go	52.58% <0.00%> (-12.12%)	⬇️

[Kubuxu]

I feel that in most cases the cert subscriber doesn't care about the complete sequence of certs, only the most recent one. I would for sure keep this branch around though as the code might be useful in the future.

[Stebalien]

Yeah, that's where I ended up. However, light clients (and lotus APIs in general) will likely want this kind of API.

[Stebalien]

Closing in favor of #646. Every time I've wanted to subscribe for certificates, I've only cared about the latest. Especially because certificates are usually stored inside the certstore anyways.