Support for 3LO in Gateway Targets or MCP on Runtime

**Which component is impacted by this improvement?**
- [x] 01-AgentCore-runtime
- [x] 02-AgentCore-gateway
- [x] 03-AgentCore-identity
- [ ] 04-AgentCore-memory
- [ ] 05-AgentCore-tools
- [ ] 06-AgentCore-observability
- [ ] 07-AgentCore-E2E

**Improvement Description**
A clear and concise description of the improvement.
```
Unfortunately, 3LO only works for internal tools exists within an Agent deployed on Runtime and doesn't work with Targets behind Gateway or MCP server deployed on Runtime. This is not much useful if 3LO can be used only within Agent internal tools. What if my Agent wants to access Confluence, Jira, Github on behalf of user (not admin oAuth or API key) but impersonate as the user accessing the agent? Do I have to create all tools within the Agent to honor 3LO? This is extremely limiting for our use case. Anybody else have a solution or facing similar issue?
```