Add support for multiple agents in agent server config

## Motivation

- To support having multiple agents defined in the agent runtime server, the user should be able to define multiple agents in the agent server configuration.

## Required Features

- Add support for defining multiple agents in the agents config TOML file
- Without breaking support for existing single-agent configurations
- Implement this by using a model of defining default configurations at root level, and each agent definitions are an override.

## Impact

- This change should not break existing configurations.

## Testing Scenarios

- Add unit tests that test validation rules for agent configs and checking whether agent overrides work as intended
- Run sample config generator to ensure definition is well formed

JIRA Issue: BA-2752