pubsub: Add Message.LoggableID

[vangent]

An opaque field useful only for debugging and logging.

Fixes #2994.

[codecov]

Codecov Report

Merging #2998 (a0a1443) into master (5f19477) will increase coverage by 0.08%.
The diff coverage is 89.13%.

@@            Coverage Diff             @@
##           master    #2998      +/-   ##
==========================================
+ Coverage   69.33%   69.42%   +0.08%     
==========================================
  Files         111      111              
  Lines       11628    11652      +24     
==========================================
+ Hits         8062     8089      +27     
+ Misses       2899     2897       -2     
+ Partials      667      666       -1     

Impacted Files	Coverage Δ
pubsub/azuresb/azuresb.go	20.00% <0.00%> (-0.10%)	⬇️
secrets/azurekeyvault/akv.go	46.39% <ø> (+2.06%)	⬆️
pubsub/awssnssqs/awssnssqs.go	83.01% <100.00%> (+0.05%)	⬆️
pubsub/gcppubsub/gcppubsub.go	75.90% <100.00%> (+0.14%)	⬆️
pubsub/kafkapubsub/kafka.go	81.96% <100.00%> (+0.45%)	⬆️
pubsub/mempubsub/mem.go	79.20% <100.00%> (+0.16%)	⬆️
pubsub/natspubsub/nats.go	72.22% <100.00%> (+0.34%)	⬆️
pubsub/pubsub.go	92.15% <100.00%> (+0.16%)	⬆️
pubsub/rabbitpubsub/rabbit.go	78.98% <100.00%> (+0.46%)	⬆️
blob/s3blob/s3blob.go	89.47% <0.00%> (+0.52%)	⬆️
... and 1 more

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