Ability to set retry behaviors from environment variable

Today when using bindings I can pop in the `%syntax%` to load in a value from environment variable. I can't do that with retry policies.

```
"retry": {
"strategy": "fixedDelay",
"maxRetryCount": 100,
"delayInterval": "%delay_interval%"
}
```

Would be good to support resolving from env.

Workaround today is to use the `host.json` file, and define the value using the `host.json` documented env path:

"AzureFunctionsJobHost__retry__delayInterval": "00:00:05"

The issue names no source file or test; start by locating retry-policy handling for Java bindings and the existing environment-variable resolution path. Compare it with the documented host.json environment path, then verify that a placeholder such as %delay_interval% is resolved for retry.delayInterval and add coverage for the requested behavior.

Written by the indexing model from the issue text.