Extract plan metadata interface

[notaphplover]

Description

• Added PlanMetadata internal interface.
• Updated interfaces.NextArgs with isOptional.

Related Issue

#566

Motivation and Context

This PR updates the planning algorithm in a way it can be requested optional activations in the root level of the plan.

How Has This Been Tested?

• Test passes successfully.

Types of changes

• Updated docs / Refactor code / Added a tests case (non-breaking change)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the changelog.

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.95%. Comparing base (b391465) to head (e38e7bc).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/container/container.ts	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1673      +/-   ##
==========================================
- Coverage   99.09%   98.95%   -0.15%     
==========================================
  Files          52       52              
  Lines        1333     1343      +10     
  Branches      181      184       +3     
==========================================
+ Hits         1321     1329       +8     
- Misses         11       13       +2     
  Partials        1        1              

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