Feature/control legend css injection

[leechoohyoung]

New Pull Request

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Summary

This PR introduces a new option to conditionally control CSS style injection for the chart legend. This change addresses potential Content Security Policy (CSP) errors that occur in environments where style-src 'unsafe-inline' is not permitted.
By adding a new option (e.g., legend.injectStyleSheet), developers can now disable the automatic injection of styles, making the library more compatible with strict CSP environments. The changes have been verified using the provided sample pages.

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• 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 not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• My branch is up to date with any changes from the main branch