feat: standardize user agent value

[AllanZhengYP]

The purpose of this change is to standardize the user agent behavior across the AWS tools. A brief summary is:

1. Only basic version information will be sent with User-Agent header: aws-sdk-js/[version];
2. More tracking metrics are sent with x-amz-user-agent header
3. User specified user agent prefix are prepended onto both headers.
4. Include required tool id and version: aws-sdk-js/[version],
5. Include required language identifier and version: lang/js,
6. Include optional language metadata: md/nodejs/[version] or md/browser/[version] or md/rn/[version]
7. Include required OS info: os/[name]/[version]
8. include optional modular SDK module version: api/[serviceId]/[version]
9. include optional client config like: cfg/retry-mode/[mode]

Here's an example UA string:
aws-sdk-js/3.0.0-rc.9 os/linux/4.14.200-155.322.amzn2.x86_64 lang/js md/nodejs/12.14.0 api/iam/3.0.0-rc.9 cfg/retry-mode/standard

In browser, I introduced bowser to make best effort infer the OS and browser version from the UA string. This is so far the best choice considering downloads, size, and GH activities.

TODO: bellows are features to be addressed in following PR:

1. Paginator, waiter and lib-storage has limitation of adding ft/[paginator, waiter, s3-transfer] ua to these requests. We need to address the issues with these features before we can add user agent for them.

A noteworthy change in this PR is that I added a UserAgent abstraction. So users don't need to deal with the string concatenation, instead they can just put the tuple by context.userAgent.push(["foo-api", "1.0.0"])

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 #1775 (0d861f7) into master (de75f7e) will decrease coverage by 0.49%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1775      +/-   ##
==========================================
- Coverage   79.77%   79.28%   -0.50%     
==========================================
  Files         325      367      +42     
  Lines       12087    15118    +3031     
  Branches     2553     3221     +668     
==========================================
+ Hits         9643    11987    +2344     
- Misses       2444     3131     +687     

Impacted Files	Coverage Δ
protocol_tests/aws-json/models/models_0.ts	79.36% <0.00%> (-20.64%)	⬇️
packages/util-user-agent-node/src/index.ts	88.88% <0.00%> (-11.12%)	⬇️
protocol_tests/aws-restjson/models/models_0.ts	89.43% <0.00%> (-10.57%)	⬇️
...rotocol_tests/aws-restxml/protocols/Aws_restXml.ts	64.10% <0.00%> (-2.15%)	⬇️
...ackages/node-http-handler/src/node-http-handler.ts	98.00% <0.00%> (-2.00%)	⬇️
packages/middleware-sdk-sqs/src/send-message.ts	86.66% <0.00%> (-0.84%)	⬇️
protocol_tests/aws-query/protocols/Aws_query.ts	66.34% <0.00%> (-0.38%)	⬇️
...kages/middleware-sdk-sqs/src/send-message-batch.ts	92.30% <0.00%> (-0.29%)	⬇️
...kages/fetch-http-handler/src/fetch-http-handler.ts	9.30% <0.00%> (-0.23%)	⬇️
protocol_tests/aws-ec2/protocols/Aws_ec2.ts	66.93% <0.00%> (-0.01%)	⬇️
... and 187 more

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[trivikr]

Revisiting this PR to check User Agent values before and after:

Node.js:

• Before: aws-sdk-nodejs-v3-@aws-sdk/client-dynamodb/1.0.0-rc.10 darwin/v14.17.1
• After: aws-sdk-js/3.1.0 os/darwin/20.5.0 lang/js md/nodejs/14.17.1 api/dynamodb/3.1.0

Browser:

• Before: aws-sdk-js-v3-@aws-sdk/client-dynamodb/1.0.0-rc.10 Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.114 Safari/537.36
• After: aws-sdk-js/3.1.0 os/macOS/10.15.7 lang/js md/browser/Chrome_91.0.4472.114 api/dynamodb/3.1.0