feat: Implement custom sharding -- iOS

[pawelpasterz]

Fixes #1665

Detailed documentation

This is the first implementation of the custom sharding feature, below assumptions were made:

1. flank does not validate provided json -- we trust the user that he/she knows what is doing
2. sharding-json is the source of truth -- no smart sharding applied
3. dump shards structure was required to be updated

Test Plan

How do we know the code works?

V1

1. build flank ./gradlew flankFullRun
2. create config yml

   gcloud:
     test: [PATH]/EarlGreyExample.zip
     xctestrun-file: [PATH]/EarlGreyExampleSwiftTests.xctestrun
   flank:
     max-test-shards: 2
     disable-results-upload: true
     disable-usage-statistics: true
   

3. run flank firebase test ios run -c=[YOUR CONFIG] --dump-shards
4. copy ios_shards.json, rename it and play around with shards, example: add another shard, move tests etc
5. add sharding-json: [PATH TO YOUR CUSTOM SHARDING JSON] to your config yaml
6. run flank firebase test ios run -c=[YOUR CONFIG] --dump-shards
7. observe new sharding in ios_shards.json
8. run flank firebase test ios run -c=[YOUR CONFIG]
9. test run should proceed and end normally with sharding that you created

V2

1. build flank ./gradlew flankFullRun
2. create config yml

   gcloud:
     test: [PATH]/FlankTestPlansExample.zip
     xctestrun-file: [PATH]/FlankTestPlansExample/AllTests.xctestrun
   flank:
     max-test-shards: 2
     disable-results-upload: true
     disable-usage-statistics: true
   

3. run flank firebase test ios run -c=[YOUR CONFIG] --dump-shards
4. copy ios_shards.json, rename it and play around with shards, example: add another shard, move tests etc
5. add sharding-json: [PATH TO YOUR CUSTOM SHARDING JSON] to your config yaml
6. run flank firebase test ios run -c=[YOUR CONFIG] --dump-shards
7. observe new sharding in ios_shards.json
8. run flank firebase test ios run -c=[YOUR CONFIG]
9. test run should proceed and end normally with sharding that you created

Checklist

• Documented
• Unit tested

[github-actions]

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[pawelpasterz]

For reference: #1736

[github-actions]

Timestamp: 2021-04-08 15:52:21
Buildscan url for ubuntu-workflow run 730109605

[Sloox]

minor wording change

[Sloox]

Minor changes.

[Sloox]

This change from ShardChunks to List<Map<...>> looks crazy. Is it worth not having another alias here like before?
maybe CustomShardChunks?
WDYT? Worth it?

[pawelpasterz]

Why not! 👍🏻
Personally, I don't like type aliases used as cover for data structures but I think I'm in the minority