Introduce Deployer as an abstraction

[jchapuis]

Formalize the notion of "deploying a repository" with actor-like semantics.

The deploy functions are simplified, with platform-specific parameters captured by an abstract type. As a result, this is however an API-breaking change.

[codecov]

Codecov Report

Merging #274 (3cc5fb8) into master (a8c5380) will increase coverage by 0.21%.
The diff coverage is 100.00%.

❗ Current head 3cc5fb8 differs from pull request most recent head d793216. Consider uploading reports for the commit d793216 to get more accurate results

@@            Coverage Diff             @@
##           master     #274      +/-   ##
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+ Coverage   89.93%   90.15%   +0.21%     
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  Files          71       71              
  Lines         914      914              
  Branches      121       98      -23     
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+ Hits          822      824       +2     
+ Misses         92       90       -2     

Files Changed	Coverage
...deploy/internal/DurableShardedEntityDeployer.scala	ø
...deploy/internal/DurableShardedEntityDeployer.scala	ø
...ala/endless/runtime/akka/deploy/AkkaDeployer.scala	100.00%
...less/runtime/akka/deploy/AkkaDurableDeployer.scala	100.00%
...y/internal/EventSourcedShardedEntityDeployer.scala	100.00%
...ka/deploy/internal/ShardedRepositoryDeployer.scala	100.00%
.../main/scala/endless/example/app/akka/AkkaApp.scala	100.00%
...ain/scala/endless/example/app/pekko/PekkoApp.scala	100.00%
...a/endless/runtime/pekko/deploy/PekkoDeployer.scala	100.00%
...ss/runtime/pekko/deploy/PekkoDurableDeployer.scala	100.00%
... and 2 more