Correct `StaticSite.viewerRequest` type

[danielsharvey]

Reference:
https://github.com/sst/sst/blob/19b85a752620151232715a720331aeabd0c1ab4b/platform/src/components/aws/static-site.ts#L126-L163

The implementation supports a full function definition as a string as well as the object.

See
https://github.com/sst/sst/blob/8dd6fa1823f620298929a188a308e6170246adbb/platform/src/components/aws/static-site.ts#L877-L878

Note that Router does not support this.