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.

Comparison is base (47e70e2) 62.39% compared to head (c03e29d) 61.89%.

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             @@
##             main     #158      +/-   ##
==========================================
- Coverage   62.39%   61.89%   -0.51%     
==========================================
  Files          49       49              
  Lines        4329     4348      +19     
==========================================
- Hits         2701     2691      -10     
- Misses       1333     1357      +24     
- Partials      295      300       +5     

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