[cmd/mdatagen] Add support for optional attribute

[sincejune]

Description

Add support for optional attribute, ref: Conditional Required Attribute

Notes:

• Add helper.tmpl for helper templates
• Add With<AttributeName>EventAttribute and With<AttributeName>MetricAttribute for optional attributes
• Add some helper functions in go files.

Link to tracking issue

Relevant to #12571

Testing

Updated

Documentation

Added

[codecov]

Codecov Report

Attention: Patch coverage is 97.16981% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.64%. Comparing base (5800834) to head (80f6fa6).

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/command.go	92.30%	0 Missing and 2 partials ⚠️
cmd/mdatagen/internal/event.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13007      +/-   ##
==========================================
+ Coverage   91.62%   91.64%   +0.01%     
==========================================
  Files         504      504              
  Lines       27888    27960      +72     
==========================================
+ Hits        25553    25624      +71     
+ Misses       1844     1843       -1     
- Partials      491      493       +2     

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[sincejune]

I added some helper functions in the Go files to simplify the templates. I'd like to remove the helper functions if possible, but it doesn't seem to be easy.