Inject value from hierarchical JSON key stored in Secrets Manager

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**Tell us about your request**
I want to inject a secret from a specific json-key in a json document stored in SecretsManager.

**Which service(s) is this request for?**
Secrets Manager + ECS (Fargate)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We have a JSON document stored in Secrets Manager:
```json
{
"key1" : {
"secret1": "secret-value-1",
"secret2": "secret-value-2",
},
"key2" : {
"secret1": "secret-value-3",
"secret2": "secret-value-4",
}
}
```

We are using CDK (C#) to create ECS Fargate containers with injected secrets using:

```csharp
var secret = Secret.FromSecretNameV2(stack, "construct-id", "secret-id");
var ecsSecret = Secret.FromSecretsManager(secret, "key2.secret1");
```

However, when we try to inject that into the containers secrets, the ECS container gives the error:
`unable to retrieve secret from asm: service call has been retried 1 time(s): retrieved secret from Secrets Manager did not contain json key key2.secret1`

**Are you currently working around this issue?**
We are currently having to store a hierarchical JSON document flattened as key/value pairs.

Review the Secrets Manager and ECS Fargate behavior described in the issue, then inspect the CDK C# Secret.FromSecretsManager entry point and AWS service documentation for supported JSON-key syntax. Reproduce the key2.secret1 case and compare it with the flattened workaround; done means a hierarchical JSON value can be injected into the container successfully.

Written by the indexing model from the issue text.