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

[Hweinstock]

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.