feat(clients): populate variants in endpoints hashes

[trivikr]

Issue

Internal JS-2908

Follow-up to #2947
Cleaned up PR of #2962

Note: This PR will be rebased from main and made ready after #2962 is merged. PR is ready!

Description

Populates variants in endpoints hashes:

• The hostname and variants are now compulsory keys for RegionHash and PartitionHash.
  • The hostname can be removed, after we’ve completely switched to variants.
• The variants array is populated with default hostname with no tags.
  • This will make utilities in config-resolver simpler, as we can ignore hostname and process just the variants.

Testing

N/A as variants are additive. The functional tests are successful.
The follow-up PRs will update unit tests in config-resolver, and affected middleware packages.

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[codecov-commenter]

Codecov Report

Merging #2974 (6d2bf78) into main (55f4e85) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2974   +/-   ##
=======================================
  Coverage   58.70%   58.70%           
=======================================
  Files         566      566           
  Lines       30315    30315           
  Branches     7460     7460           
=======================================
  Hits        17796    17796           
  Misses      12519    12519           

Impacted Files	Coverage Δ
...lients/client-lex-runtime-service/src/endpoints.ts	100.00% <ø> (ø)
clients/client-mediastore-data/src/endpoints.ts	100.00% <ø> (ø)
clients/client-s3-control/src/endpoints.ts	100.00% <ø> (ø)
clients/client-sts/src/endpoints.ts	100.00% <ø> (ø)
private/aws-protocoltests-ec2/src/endpoints.ts	100.00% <ø> (ø)
private/aws-protocoltests-json-10/src/endpoints.ts	100.00% <ø> (ø)
private/aws-protocoltests-json/src/endpoints.ts	100.00% <ø> (ø)
private/aws-protocoltests-query/src/endpoints.ts	100.00% <ø> (ø)
...rivate/aws-protocoltests-restjson/src/endpoints.ts	100.00% <ø> (ø)
private/aws-protocoltests-restxml/src/endpoints.ts	100.00% <ø> (ø)
... and 10 more

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