fix(fetch-secrets): swap fetch secret action to use node instead of bash

[Hweinstock]

Problem

The fetch secrets action does not work on windows runners because it relies on bash.

Solution

Swap to node scripts instead of inline bash scripts for the secret id to env var mapping logic + register mask logic.

Testing

ran the script against the account hosting the secrets myself:

$ output_file=$(mktemp) && env SHARED_NAMES=SLACK_OPEN_PRS_WEBHOOK_URL REPO_NAMES=AWS_ROLE_ARN CALLER_REPO=aws/agentcore-cli GITHUB_OUTPUT="$output_file" node /[repo-path]/.github/actions/fetch-secrets/build-secret-mappings.js && cat "$output_file"
Resolved secret mappings:
  SLACK_OPEN_PRS_WEBHOOK_URL,shared/SLACK_OPEN_PRS_WEBHOOK_URL
  AWS_ROLE_ARN,aws/agentcore-cli/AWS_ROLE_ARN
mappings<<SECRET_MAPPINGS_EOF
SLACK_OPEN_PRS_WEBHOOK_URL,shared/SLACK_OPEN_PRS_WEBHOOK_URL
AWS_ROLE_ARN,aws/agentcore-cli/AWS_ROLE_ARN
SECRET_MAPPINGS_EOF

Note: all consumers leverage pinned commits, so this will not break them.

Next Steps

Once this is merged, I can make a commit to the agentcore-cli repo to use the new version and enable windows on e2e tests.