Add support for composite unique constraint in auto migration

[atkei]

Related to #172, #175 and based on #957.

Add Constraint and UniqueConstraint to support composite unique constraint in auto migraiton.

Some work remains such as adding doc and testing, but I would like to have feedback on my proposal first.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.85253% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 89.72%. Comparing base (363d683) to head (07f8b94).
Report is 1 commits behind head on master.

Files	Patch %	Lines
piccolo/apps/migrations/auto/schema_differ.py	92.85%	3 Missing ⚠️
piccolo/apps/migrations/auto/migration_manager.py	97.36%	2 Missing ⚠️
piccolo/apps/migrations/auto/diffable_table.py	95.00%	1 Missing ⚠️
piccolo/apps/migrations/auto/schema_snapshot.py	80.00%	1 Missing ⚠️
piccolo/constraint.py	96.96%	1 Missing ⚠️
piccolo/table.py	90.90%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #984      +/-   ##
==========================================
- Coverage   92.78%   89.72%   -3.07%     
==========================================
  Files         108      109       +1     
  Lines        8182     8399     +217     
==========================================
- Hits         7592     7536      -56     
- Misses        590      863     +273     

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[sinisaos]

@dantownsend I hope it's not a problem, but I have approved and run the workflow for this PR. I have already tried these changes in the @atkei local branch and in my case everything works great as I wrote in this comment. Can you try and review this PR. Thanks.

[dantownsend]

@sinisaos Awesome thanks - feel free to run the CI whenever you want.

@atkei Thanks a lot for this - looks great!

[AlexanderMakarov]

@sinisaos @dantownsend - could you make a hint - when it would be merged? Would it mean new version/release?

[dantownsend]

@AlexanderMakarov I'll try my best to review it properly this week.

[MoSheikh]

Currenty looking for this funtionality - would appreciate it someone on the team would be able to review!