feat: Support Secrets Manager references in runtime environment variables

## Problem

The environment_variables block on AgentCore Agent Runtime only accepts plaintext values. There is no mechanism to reference secrets from AWS Secrets Manager (or SSM Parameter Store) that get resolved at container start time.
This is a gap compared to other AWS container services. For example, ECS task definitions support valueFrom on container environment variables, which resolves Secrets Manager ARNs or SSM Parameter Store paths when the container starts — without ever exposing the secret value in the task definition, Terraform state, or the AWS console.

## Use Case

When integrating third-party services that require credentials via environment variables (observability platforms, external APIs, etc.), the current options are:

1. Plaintext env vars — credentials are visible in Terraform state, terraform plan output, the AgentCore console, and the AWS API. This is undesirable for production workloads with compliance requirements.
2. Runtime secret fetching — application code fetches secrets from Secrets Manager at startup and sets env vars programmatically. This works but introduces timing issues when platform-level components (e.g., ADOT auto-instrumentation) read environment variables before application code has a chance to set them.

Neither option provides the same security and ergonomics that ECS users already have.

## Proposed Solution

Support an optional secret reference syntax in environment_variables that resolves Secrets Manager (and ideally SSM Parameter Store) values at container start, before any application code or auto-instrumentation runs. For example:

```python
# Pseudocode
environment_variables = {
"MY_API_KEY": {"secret_arn": "arn:aws:secretsmanager:us-east-1:123456789:secret:my-key"},
"SOME_CONFIG": "plaintext-value", # existing behavior preserved }
```

This would:
- Keep secrets out of Terraform state and API responses
- Ensure secrets are available as env vars before any process (including auto-instrumentation) starts
- Align with the ECS valueFrom pattern that AWS users are already familiar with

No implementation files or tests are named. Start by locating the AgentCore Agent Runtime environment_variables schema and related tests, then compare the proposed secret_arn shape with the ECS valueFrom pattern; done means plaintext values still work and supported Secrets Manager or SSM references resolve before the container starts without exposing secret values.

Written by the indexing model from the issue text.