feat: use 2 nodemailer clients

[wanlingt]

Problem

Closes #126

Solution

• Create 2 nodemailer clients - 1 using SG env vars, another using US env vars
• Conditionally send mail from SG client depending on a rollout threshold NODEMAILER_THRESHOLD_SG

Breaking Changes

• No - this PR is backwards compatible

Tests

• Before sending emails: check the SES dashboard (US and SG) to see the count of emails sent. Trigger the sending of multiple emails (e.g. OTPs). These emails should be sent successfully. Check the cloudwatch logs to determine which nodemailer client is being used. Check the SES dashboard (US and SG) to see if the new count of emails tallies with the cloudwatch logs.

fields @timestamp, @message
| filter label like 'mail'
| sort @timestamp desc

Deploy Notes

New environment variables:

• SES_PORT_US
• SES_HOST_US
• SES_USER_US
• SES_PASS_US
• SES_PORT_SG
• SES_HOST_SG
• SES_USER_SG
• SES_PASS_SG
• NODEMAILER_SG_WARMUP_START_DATE: Date to start rollout of emails from SG nodemailer client. Should be in the format YYYY-MM-DDTHH:MM:SS+08:00

The permissions for the SES sending policy should also be modified accordingly to allow both US and SG nodemailer clients to send emails

[tshuli]

lgtm, other than Tim's comments and a minor nit

[timotheeg]

LGTM 👍