elasticache: Serverless cache attr_endpoint returning aws_cdk.Reference

### Describe the bug

The `attr_endpoint` attribute of the `CfnServerlessCache` should be a string or be resolvable to a string but instead it's `aws_cdk.Reference` instead.

### Expected Behavior

I expected that this script should be able to run:
```
import aws_cdk as cdk
from aws_cdk import (
Stack,
aws_elasticache as elasticache,
aws_ssm as ssm
)
from constructs import Construct
import os

class test_stack(Stack):

def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
super().__init__(scope, construct_id, **kwargs)

self.redis_cluster = elasticache.CfnServerlessCache(
self,
"RedisCluster",
engine="redis",
serverless_cache_name="ServerlessCache",
)

self.redis_endpoint = ssm.StringParameter(
self,
"redis-endpoint",
parameter_name="redis_endpoint",
string_value=self.redis_cluster.attr_endpoint
)

if __name__ == "__main__":
app = cdk.App()
test_stack(app, "test",
env=cdk.Environment(
account=os.getenv('CDK_DEFAULT_ACCOUNT'),
region=os.getenv('CDK_DEFAULT_REGION')
))
app.synth()
```

### Current Behavior

Instead of running successfully this traceback is returned:
Traceback (most recent call last):
File "/Users/home/test.py", line 32, in
test_stack(app, "test",
File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/jsii/_runtime.py", line 118, in __call__
inst = super(JSIIMeta, cast(JSIIMeta, cls)).__call__(*args, **kwargs)
File "/Users/home/test.py", line 22, in __init__
self.redis_endpoint = ssm.StringParameter(
File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/jsii/_runtime.py", line 118, in __call__
inst = super(JSIIMeta, cast(JSIIMeta, cls)).__call__(*args, **kwargs)
File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/aws_cdk/aws_ssm/__init__.py", line 8397, in __init__
props = StringParameterProps(
File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/aws_cdk/aws_ssm/__init__.py", line 8924, in __init__
check_type(argname="argument string_value", value=string_value, expected_type=type_hints["string_value"])
File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/typeguard/__init__.py", line 785, in check_type
raise TypeError(
TypeError: type of argument string_value must be str; got aws_cdk.Reference instead

### Reproduction Steps

Running this script below should reproduce the bug:
```
import aws_cdk as cdk
from aws_cdk import (
Stack,
aws_elasticache as elasticache,
aws_ssm as ssm
)
from constructs import Construct
import os

class test_stack(Stack):

def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
super().__init__(scope, construct_id, **kwargs)

self.redis_cluster = elasticache.CfnServerlessCache(
self,
"RedisCluster",
engine="redis",
serverless_cache_name="ServerlessCache",
)

self.redis_endpoint = ssm.StringParameter(
self,
"redis-endpoint",
parameter_name="redis_endpoint",
string_value=self.redis_cluster.attr_endpoint
)

if __name__ == "__main__":
app = cdk.App()
test_stack(app, "test",
env=cdk.Environment(
account=os.getenv('CDK_DEFAULT_ACCOUNT'),
region=os.getenv('CDK_DEFAULT_REGION')
))
app.synth()
```

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### CDK CLI Version

2.113.0

### Framework Version

_No response_

### Node.js Version

v18.2.0

### OS

MacOS 13.5.2

### Language

Python

### Language Version

3.10

### Other information

_No response_

Start at the CfnServerlessCache.attr_endpoint entry point and compare its value handling with the ssm.StringParameter string_value input. Run the provided Python reproduction and synthesize the stack to observe the failure. Done means attr_endpoint is accepted as a string or string-resolvable value without the reported type error.

Written by the indexing model from the issue text.