feat: add an "unregister" function for observers

[Stebalien]

No description provided.

[codecov]

Codecov Report

Merging #8445 (895e770) into master (f378c6d) will increase coverage by 6.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #8445      +/-   ##
==========================================
+ Coverage   34.56%   40.62%   +6.06%     
==========================================
  Files         681      686       +5     
  Lines       75174    75413     +239     
==========================================
+ Hits        25984    30637    +4653     
+ Misses      44340    39471    -4869     
- Partials     4850     5305     +455     

Impacted Files	Coverage Δ
chain/events/observer.go	66.20% <0.00%> (-0.22%)	⬇️
chain/events/message_cache.go	87.50% <0.00%> (-12.50%)	⬇️
extern/sector-storage/sched_worker.go	79.41% <0.00%> (-0.59%)	⬇️
chain/types/blockheader.go	55.55% <0.00%> (ø)
lib/nullreader/reader.go	0.00% <0.00%> (ø)
journal/mockjournal/journal.go	71.42% <0.00%> (ø)
chain/actors/builtin/paych/mock/mock.go	71.42% <0.00%> (ø)
chain/vectors/gen/main.go	2.36% <0.00%> (ø)
markets/dagstore/mocks/mock_lotus_accessor.go	80.48% <0.00%> (ø)
extern/sector-storage/mock/mock.go	60.20% <0.00%> (+0.25%)	⬆️
... and 162 more