aws-cloudfront: SecretValue not resolved inside Fn.base64() passed to FunctionCode

### Describe the bug

When creating a CloudFront Function for simple HTTP basic authentication, you can insert a SecretValue into the inline FunctionCode. But if you wrap the SecretValue into Fn.base64(), the actually deployed value is not the resolved SecretValue, but a base64-encoded reference string that looks like `{{resolve:secretsmanager:...`.

In the case of HTTP basic auth, we need to pass a base64-encoded value that contains `username:password`, where username and password come from Secrets Manager. The workaround is to do the base64-encoding at runtime inside the CloudFront function, but it would be clearer to encode it at deployment time with Fn.base64().

### Expected Behavior

Passing a SecretValue wrapped with Fn.base64() should resolve into the base64-encoded secret value.

### Current Behavior

Passing a SecretValue wrapped with Fn.base64() is deployed as a base64-encoded unresolved `{{resolve:secretsmanager:...` string.

### Reproduction Steps

Untested quick reference code:
```
const username = SecretValue.secretsManager(props.secretName, { jsonField: 'username' }).unsafeUnwrap();
const password = SecretValue.secretsManager(props.secretName, { jsonField: 'password' }).unsafeUnwrap();
const authValue = `${username }:${password }`;
const basicAuthFunction = new CloudFrontFunction(this, 'BasicAuthFunction', {
code: FunctionCode.fromInline(`
...
if (event.request.headers.authorization && event.request.headers.authorization.value === '${Fn.base64(authValue)}')
...
`),
});
```

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.40.0 (build 56ba2ab)

### Framework Version

_No response_

### Node.js Version

v16.13.1

### OS

Windows 11 WSL2

### Language

Typescript

### Language Version

_No response_

### Other information

_No response_

Start by tracing how Fn.base64() handles the SecretValue tokens used by SecretValue.secretsManager(), then follow their use in FunctionCode.fromInline() for CloudFrontFunction. Reproduce the issue with the provided basic-auth example and verify that deployment produces a base64-encoded resolved secret rather than an encoded Secrets Manager reference.

Written by the indexing model from the issue text.