Terraform not allowing to pass nested values (like assume_role.role_arn) to the backend.s3 configuration using cli (-backend-config="KEY=VALUE") and config file (-backend-config=PATH)

### Terraform Version

```shell
1.11.4
```

### Terraform Configuration Files

```terraform
terraform {
required_providers {
aws = {
source = "hashicorp/aws"
version = ">= 4.33.0"
}
}

backend "s3" {
bucket = ""
use_lockfile=""
key=""
region=""
assume_role = {
role_arn = ""
external_id = ""
}
}
required_version = ">= 0.14.9"
}
...terraform config...
```

### Debug Output

not applicable

### Expected Behavior
```
Terraform has been successfully initialized!

You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.

If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.
```
### Actual Behavior
```
Initializing the backend...
╷
│ Error: Invalid backend configuration argument
│
│ The backend configuration argument "assume_role.role_arn" given on the command line is not expected for the selected backend type.
╵
╷
│ Error: Invalid backend configuration argument
│
│ The backend configuration argument "assume_role.external_id" given on the command line is not expected for the selected backend type.
```
### Steps to Reproduce
```
terraform -chdir=.iac init \
-backend=true \
-backend-config="bucket=my-humble-bucket" \
-backend-config="key=my-humble-key.tfstate" \
-backend-config="region=af-south-1" \
-backend-config="use_lockfile=true" \
-backend-config="assume_role.role_arn=arn:aws:iam::123456789012:role/my-humble-role" \
-backend-config="assume_role.external_id=my-humble-id" \
-force-copy
```
### Additional Context

I'm having the same issue when i true to use those exact same values on backend.config file, trying to pass with the `-backend-config=PATH` flag...

I noticed this issue after upgrading the Terraform version in our team's Continuous Integration tool from **v1.8.5** to **v1.11.4**. Previously, we were able to provide the `role_arn ` programmatically because it was a top-level attribute in the `backend.s3` block. Now that it has to be nested under `assume_role`, Terraform throws an error when we try to pass it dynamically.

Edit:
I've found out this still works when using a `.hcl` file, passing a value like this one:

```
bucket = "my-humble-bucket"
key="test/my-humble-key.tfstate"
region="af-south-1"
use_lockfile="true"
assume_role= {
role_arn = "arn:aws:iam::123456789012:role/my-humble-role"
external_id="my-humble-id"
}
```

### References

_No response_

### Generative AI / LLM assisted development?

_No response_