Add typescript to `apiSigning.js`

[vicky-g]

Description

One of our goals is to add typescript to Content Node code base. Here is a start: adding typescript to apiSigning.js

Tests

No functionality changes, just converting to typescript

Monitoring - How will this change be monitored? Are there sufficient logs / alerts?

If requests verification requests start failing, look into api signing

[vicky-g]

@jonaylor89