feat: Change message id on PubsubMessages to be an encoded MessageMetadata and rename PublishMetadata to MessageMetadata

[dpcollins-google]

PublishMetadata is both the partition and offset, and is an ID suitable for deduplication.

Fixes #471

[codecov]

Codecov Report

Merging #482 (8a6fec5) into master (c709bef) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #482      +/-   ##
============================================
+ Coverage     75.31%   75.38%   +0.06%     
- Complexity      758      760       +2     
============================================
  Files           129      129              
  Lines          3865     3875      +10     
  Branches        174      174              
============================================
+ Hits           2911     2921      +10     
  Misses          850      850              
  Partials        104      104              

Impacted Files	Coverage Δ	Complexity Δ
...ublite/cloudpubsub/internal/WrappingPublisher.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
.../wire/PartitionCountWatchingPublisherSettings.java	100.00% <ø> (ø)	3.00 <0.00> (ø)
...sublite/internal/wire/RoutingPublisherBuilder.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...internal/wire/SinglePartitionPublisherBuilder.java	100.00% <ø> (ø)	2.00 <0.00> (ø)
...a/com/google/cloud/pubsublite/MessageMetadata.java	100.00% <100.00%> (ø)	5.00 <2.00> (?)
...loud/pubsublite/cloudpubsub/MessageTransforms.java	100.00% <100.00%> (ø)	18.00 <2.00> (+2.00)
...oud/pubsublite/cloudpubsub/SubscriberSettings.java	59.42% <100.00%> (+0.59%)	12.00 <0.00> (ø)
...internal/wire/PartitionCountWatchingPublisher.java	97.05% <100.00%> (ø)	18.00 <2.00> (ø)
...oud/pubsublite/internal/wire/RoutingPublisher.java	95.45% <100.00%> (ø)	6.00 <4.00> (ø)
...ublite/internal/wire/SinglePartitionPublisher.java	100.00% <100.00%> (ø)	5.00 <1.00> (ø)

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[anguillanneuf]

LGTM.

You don't really need to change README.md as any change in .readme-partials.yaml will trigger an update in README.md.