fix(prover): Guardian prover sign wait

[cyberhorsey]

wait for chain head to have block before trying to sign. THis is perhaps a temporary fix as there should be a secondary mechanism for guardian prover signing blocks, perhaps with its own blockProposedCh and own iterators, that way it doesnt exist alongside proving code.

I will do a further PR for a GuardianProverSigner struct that embeds in the Prover, and has its own event loop, after the weekend, if we want to go that route.

[codecov]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (d632109) 60.67% compared to head (94f7e5a) 60.53%.

Files	Patch %	Lines
prover/prover.go	35.48%	15 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
- Coverage   60.67%   60.53%   -0.14%     
==========================================
  Files          45       45              
  Lines        5416     5440      +24     
==========================================
+ Hits         3286     3293       +7     
- Misses       1800     1813      +13     
- Partials      330      334       +4     

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