Partitions: Backends of all global services are now partitioned

[bblommers]

Related to #7699

Regional services have their data stored in a BackendDict[ACCOUNT_ID][REGION] Global services (like S3) would use a magic global key instead: BackendDict[ACCOUNT_ID]["global"]

These services are not truly global though - data in the regular AWS partition is separate from the data in the AWS China/AWS GOV/etc partitions. This PR changes all references to the magic global to use the partition name instead: BackendDict[ACCOUNT_ID][aws|aws-cn|aws-gov|etc]

Existing users will sometimes import backends manually, and also refer to BackendDict[ACCOUNT_ID]["global"]. This will now resolve to BackendDict[ACCOUNT_ID]["aws"] for backwards compatibility.

The Moto API still uses a single global backend. Operations in the Moto API either call a specific service, or deal with configuration issues - there is no point in partitioning either.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.36957% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 94.33%. Comparing base (303a6dd) to head (88d70c4).

Files	Patch %	Lines
moto/moto_proxy/proxy3.py	0.00%	2 Missing :warning:
moto/iam/models.py	96.87%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7710   +/-   ##
=======================================
  Coverage   94.33%   94.33%           
=======================================
  Files        1087     1087           
  Lines       92765    92819   +54     
=======================================
+ Hits        87508    87563   +55     
+ Misses       5257     5256    -1     

Flag	Coverage Δ
servertests	29.47% <33.15%> (+0.01%)	:arrow_up:
unittests	94.30% <98.36%> (+<0.01%)	:arrow_up:

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[github-actions[bot]]

This is now part of moto >= 5.0.8.dev14