Add a sequence number to advertisements

[gammazero]

The sequence number is optional to allow backwards compatability with previous existing advertisements that do not have a sequence number.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (release-v0.5.x@d416513). Click here to learn what that means.

Files	Patch %	Lines
ingest/schema/envelope.go	62.50%	4 Missing and 2 partials ⚠️
ingest/schema/types.go	0.00%	5 Missing ⚠️

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Additional details and impacted files

@@                Coverage Diff                @@
##             release-v0.5.x     #159   +/-   ##
=================================================
  Coverage                  ?   60.48%           
=================================================
  Files                     ?       57           
  Lines                     ?     5051           
  Branches                  ?        0           
=================================================
  Hits                      ?     3055           
  Misses                    ?     1664           
  Partials                  ?      332           

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