v3.1.5: Update readme, introduce support for new openapi subdomain, bump dependencies post audit

What's Changed

• Update README badges by @JJ-Cro in #189
• feat(v3.1.5): update per release notes by @JJ-Cro in #188

Full Changelog: v3.1.4...v3.1.5

View all our OKX JavaScript SDK releases here: https://siebly.io/releases?sdk=okx-api