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.