fix(clients): populate sdkId in serviceId and default to use arnNamespace as signingName

[AllanZhengYP]

Follow up to #1765
Description of changes:
The previous change #1765 put arnNamespace in serviceId client config, and use it as fallback when a service doesn't specify a signing name in its endpoints.json credentialScope. It fixes the issue but the serviceId config is not correct. Right now the serviceId is generated from arnNamespace in the Smithy model, but it can be different from the actual service id. It may result unexpected results in metrics.

This change moves the fallback-to-arnNamespace logic to endpoints.ts: every service has arnNamespace as default signingName. If they have a different signing name in its endpoints.json credentialScope, they will use that instead. Now serviceId client config can be changed to actual service id because it doesn't need to provide the signing name information.

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[codecov-io]

Codecov Report

Merging #1786 (267d32d) into master (de75f7e) will decrease coverage by 1.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1786      +/-   ##
==========================================
- Coverage   79.77%   78.72%   -1.06%     
==========================================
  Files         325      362      +37     
  Lines       12087    15301    +3214     
  Branches     2553     3333     +780     
==========================================
+ Hits         9643    12046    +2403     
- Misses       2444     3255     +811     

Impacted Files	Coverage Δ
protocol_tests/aws-json/models/models_0.ts	79.36% <0.00%> (-20.64%)	⬇️
protocol_tests/aws-restjson/models/models_0.ts	89.43% <0.00%> (-10.57%)	⬇️
...tocol_tests/aws-restxml/commands/XmlMapsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ocol_tests/aws-restxml/commands/XmlBlobsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ocol_tests/aws-restxml/commands/XmlEnumsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ocol_tests/aws-restxml/commands/XmlListsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...col_tests/aws-restjson/commands/JsonMapsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ol_tests/aws-restjson/commands/JsonBlobsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ol_tests/aws-restjson/commands/JsonEnumsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
...ol_tests/aws-restjson/commands/JsonListsCommand.ts	95.65% <0.00%> (-4.35%)	⬇️
... and 178 more

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