Using custom logger type causes panic at runtime

ref: [main/client.go](https://github.com/hashicorp/go-retryablehttp/blob/main/client.go#L458-L464)

Hey all. Can we Update the `retryablehttp.Client.Logger` field to be one of the following?

1. A composite interface (ala go 1.18+ generics)?

```
type RetryableHttpLoggerConstraint interface {
retryablehttp.Logger | retryablehttp.LeveledLogger
}
func SetLogger[T LoggerConstraint](client *retryablehttp.Client, logger T) {
client.Logger = logger
}
```

2. A single interface (`retryablehttp.LeveledLogger` __*or*__ `retryablehttp.Logger` __*or*__ something else)

Doing a type check and panic-ing at runtime is no bueno and is likely to cause incidents in production (like it did with us).