Multi-tenancy support

Although the current system is designed as a single-tenant system ([doc](https://github.com/aws-samples/remote-swe-agents/blob/c2013e8a2fb40220088fe79beef237481c0708c7/README.md#access-control-or-tenant-isolation-model)), it would be great if we could share a single RmoteSWE instance with multiple orgnizations, to reduce management cost.

We will need:

1. tenant management gui
2. add tanantIds to the dynamodb tables PK.
3. Manage credentials (e.g. github PAT) per tenant

Start with the README access-control or tenant-isolation section linked in the issue, then map how tenant management, DynamoDB table keys, and credentials are currently handled. The work is done when organizations can share one instance with isolated tenant data and per-tenant credentials, with a tenant management GUI covering the listed requirements.

Written by the indexing model from the issue text.