fix(clients): merge default variants by comparing tags

[trivikr]

Issue

Fixes #3043

Description

Merges variants from partition defaults and service defaults using tags as unique key

Testing

Verified that endpoint resolution returns dynamodb.us-gov-east-1.api.aws for DynamoDB with region=us-gov-east-1 and useDualstackEndpoint=true

Code

import { DynamoDB } from "../aws-sdk-js-v3/clients/client-dynamodb/dist-cjs/index.js";

const client = new DynamoDB({
  region: "us-gov-east-1",
  useDualstackEndpoint: true,
});
await client.listTables({});

Output

node:internal/process/esm_loader:94
    internalBinding('errors').triggerUncaughtException(
                              ^

Error: getaddrinfo ENOTFOUND dynamodb.us-gov-east-1.api.aws
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (node:dns:71:26) {
  errno: -3008,
  code: 'ENOTFOUND',
  syscall: 'getaddrinfo',
  hostname: 'dynamodb.us-gov-east-1.api.aws',
  '$metadata': { attempts: 1, totalRetryDelay: 0 }
}

---

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[codecov-commenter]

Codecov Report

Merging #3044 (4e5d0fa) into main (bcfb45b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3044   +/-   ##
=======================================
  Coverage   58.76%   58.76%           
=======================================
  Files         567      567           
  Lines       30360    30360           
  Branches     7482     7482           
=======================================
  Hits        17841    17841           
  Misses      12519    12519           

Impacted Files	Coverage Δ
...lients/client-lex-runtime-service/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)

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[github-actions]

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