Add NullableBool function (#1322)

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## Motivation

The `AppPortalAccess` function in the golang SDK takes a `ReadOnly`
parameter, of type `NullableBool`. However, the `NullableBool` struct
and associated functions aren't accessible outside the SDK, so it's not
possible to construct a `NullableBool` to pass into this function.

For other types (`NullableString`, `NullableInt32`), there's a set of
wrapper functions defined in the `svix` package.

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## Solution
Add a `NullableBool` function to the svix package, next to the existing
wrapper functions.

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go/svix.go

@@ -47,6 +47,9 @@ func NullableInt32(num *int32) *openapi.NullableInt32 {
 func Int32(i int32) *int32 {
 	return &i
 }
+func NullableBool(b *bool) *openapi.NullableBool {
+	return openapi.NewNullableBool(b)
+}
 
 func New(token string, options *SvixOptions) *Svix {
 	conf := openapi.NewConfiguration()