feat(core): set up proxy to host custom ui assets if available

[charIeszhao]

Summary

Set up proxy rules for those who uploaded custom UI assets for their sign-in experience. The proxy will host the uploaded html assets instead of the pre-built experience UI.

Testing

Locally tested

Checklist

• [ ] .changeset
• [x] unit tests
• [ ] integration tests
• [x] necessary TSDoc comments

[github-actions[bot]]

COMPARE TO master

Total Size Diff :chart_with_upwards_trend: +6.66 KB

Diff by File

|Name|Diff| |---|---| |packages/core/src/middleware/koa-serve-custom-ui-assets.test.ts|:chart_with_upwards_trend: +3.08 KB| |packages/core/src/middleware/koa-serve-custom-ui-assets.ts|:chart_with_upwards_trend: +1.67 KB| |packages/core/src/middleware/koa-spa-proxy.test.ts|:chart_with_upwards_trend: +1.02 KB| |packages/core/src/middleware/koa-spa-proxy.ts|:chart_with_upwards_trend: +649 Bytes| |packages/core/src/tenants/Tenant.ts|:chart_with_upwards_trend: +247 Bytes|