Support `Default` in Ephemeral Resources

### Module version

```
1.14.1
```

### Use-cases

vault resources often have default mount names, would like to leverage those and reduce user input

```hcl
ephemeral "vault_terraform_team_token" "test" {
mount = "terraform" # "terraform" should be the default
role_name = "test"
}
```

### Attempted Solutions

### Proposal

### References

- https://github.com/hashicorp/terraform-plugin-framework/blob/main/ephemeral/schema/string_attribute.go#L35