refactor: Rename outbox message types

[bstasyszyn]

Signed-off-by: Bob Stasyszyn Bob.Stasyszyn@securekey.com

[codecov]

Codecov Report

Merging #1249 (2347411) into main (bcc42e4) will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main    #1249      +/-   ##
==========================================
- Coverage   87.96%   87.95%   -0.02%     
==========================================
  Files         173      173              
  Lines       17364    17364              
==========================================
- Hits        15274    15272       -2     
- Misses       1330     1332       +2     
  Partials      760      760              

Impacted Files	Coverage Δ
pkg/activitypub/service/outbox/outbox.go	89.77% <80.00%> (ø)
cmd/orb-server/startcmd/start.go	68.68% <0.00%> (-0.23%)	⬇️
pkg/observer/pubsub.go	94.49% <0.00%> (ø)

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