feat: custom header by passing html element

[x] It's submitted to right branch according to our branching model
[x] It's right issue type on title
[ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
[x] The commit message follows our guidelines
[x] Tests for the changes have been added (for bug fixes/features)
[x] Docs have been added/updated (for bug fixes/features)
[x] It does not introduce a breaking change or has description for the breaking change

Added custom header functionality.
- You can set a variety of HTML elements in the header, including images, icons, and more.

To-Be

Thank you for your contribution to TOAST UI product. 🎉 😘 ✨

nhn / tui.grid