Feat: support specular intensity and F0 color in dielectric material

[zhuxudong]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Feature.

What is the current behavior? (You can also link to an open issue here)

KHR_materials_specular is not supported and F0 & F90 can't be modified in dielectric material.

What is the new behavior (if this is a feature change)?

Supported.

Other information:

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (dev/0.7@5086d41). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff             @@
##             dev/0.7     #697   +/-   ##
==========================================
  Coverage           ?   43.83%           
==========================================
  Files              ?      410           
  Lines              ?    21605           
  Branches           ?     2991           
==========================================
  Hits               ?     9470           
  Misses             ?    12135           
  Partials           ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 5086d41...4b0cd6a. Read the comment docs.