feat: add a way to see valid options for the `agentcore config` command.

### Problem
Using `agentcore config [key] [value]`, there is no way to know what the valid key value pairs are without looking at the schema in the source code

### Potential Solutions
- Display a list of valid options on the help page.
- show all keys (even unset ones) when printing config.
- link to documentation that describes the valid pairings.