fix: polygon support

[HJunyuan]

Context

• /email and /verify does not work for Polygon-issued documents

What does this PR do?

• Create a shared verify() to be used by the various lambda functions
• Auto switch between Ethereum and Polygon dependent on document.network field (see reference implementation)

[seed-deploy[bot]]

View stack outputs

- **mail-pr161** Name | Value -- | -- ServiceEndpoint | https://6k6bi7fqvl.execute-api.ap-southeast-1.amazonaws.com/pr161 - **storage-pr161** Name | Value -- | -- ServiceEndpoint | https://296alinmxd.execute-api.ap-southeast-1.amazonaws.com/pr161 - **verify-pr161** Name | Value -- | -- ServiceEndpoint | https://26z4ypagn4.execute-api.ap-southeast-1.amazonaws.com/pr161