fix(clients): default region and credential provider

[AllanZhengYP]

This change makes default credential only resolves at signer signing a request. So that it won't resolve at client instantiation. It fixes the issues that the default credentials provider resolves when credentials is not required for given operations, like CognitoIdentity.getId().

Fixes: #1798

Example stack trace for missing credentials in browsers:

Error: Credential is missing
    at SignatureV4.credentialProvider (/home/ec2-user/workspace/aws-sdk-js-v3/clients/client-cognito-identity/runtimeConfig.browser.ts:24:67)
    at SignatureV4.signRequest (/home/ec2-user/workspace/aws-sdk-js-v3/packages/signature-v4/src/SignatureV4.ts:220:36)
    at SignatureV4.sign (/home/ec2-user/workspace/aws-sdk-js-v3/packages/signature-v4/src/SignatureV4.ts:171:19)
    at /home/ec2-user/workspace/aws-sdk-js-v3/packages/middleware-signing/src/middleware.ts:28:31
    at StandardRetryStrategy.retry (/home/ec2-user/workspace/aws-sdk-js-v3/packages/middleware-retry/src/defaultStrategy.ts:125:38)
    at /home/ec2-user/workspace/aws-sdk-js-v3/packages/middleware-logger/src/loggerMiddleware.ts:21:20 {
  '$metadata': { attempts: 1, totalRetryDelay: 0 }
}

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #1834 (9a47d48) into master (f2a47e8) will increase coverage by 0.01%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #1834      +/-   ##
==========================================
+ Coverage   79.30%   79.31%   +0.01%     
==========================================
  Files         368      368              
  Lines       15132    15140       +8     
  Branches     3222     3226       +4     
==========================================
+ Hits        12001    12009       +8     
  Misses       3131     3131              

Impacted Files	Coverage Δ
packages/util-waiter/src/waiter.ts	100.00% <ø> (ø)
...middleware-user-agent/src/user-agent-middleware.ts	93.10% <66.66%> (ø)
packages/invalid-dependency/src/invalidProvider.ts	100.00% <100.00%> (ø)
packages/util-waiter/src/createWaiter.ts	100.00% <100.00%> (+11.76%)	⬆️
packages/util-waiter/src/poller.ts	100.00% <100.00%> (ø)
packages/util-waiter/src/utils/validate.ts	100.00% <100.00%> (ø)
protocol_tests/aws-json/endpoints.ts	81.48% <100.00%> (ø)
lib/storage/src/data-chunk/readable-helper.ts	88.23% <0.00%> (-4.08%)	⬇️
protocol_tests/aws-ec2/endpoints.ts	81.48% <0.00%> (ø)
... and 10 more

---

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[alexforsyth]

LGTM!

[trivikr]

Fix verified in browser using aws-sdk-js-tests

[github-actions]

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