feat: support adding items with custom attributes.

[zhu-xiaowei]

• PR title and description conform to Pull Request guidelines.

Description of changes:

1. support adding items with custom attributes.

NOTE: This is a breaking change, only pipelines from version 1.1+ can handle items with custom attribute.

How did you test these changes?
added test case to cover all item error code.

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (5d31535) 86.94% compared to head (eb0503f) 87.17%.

Files	Patch %	Lines
...ws/solution/clickstream/client/AnalyticsEvent.java	71.42%	3 Missing and 3 partials ⚠️
...ware/aws/solution/clickstream/ClickstreamItem.java	83.33%	4 Missing ⚠️
...aws/solution/clickstream/AWSClickstreamPlugin.java	77.77%	0 Missing and 2 partials ⚠️
.../aws/solution/clickstream/client/EventChecker.java	98.46%	2 Missing ⚠️
...aws/solution/clickstream/ClickstreamAnalytics.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #45      +/-   ##
============================================
+ Coverage     86.94%   87.17%   +0.23%     
- Complexity      422      447      +25     
============================================
  Files            40       42       +2     
  Lines          1501     1638     +137     
  Branches        131      149      +18     
============================================
+ Hits           1305     1428     +123     
- Misses          123      133      +10     
- Partials         73       77       +4     

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