Validate cursor for feeds

[ecooper]

This PR adds a validation check for cursor and makes sure it's a string at runtime. Non-string values can produce vague error messages otherwise.

Description

This PR follows the current client configuration validation pattern to make sure cursor is a string when it is set.

Motivation and context

While testing this change internally, we discovered if you managed to pass a non-string cursor during runtime, the error returned wasn't very helpful. It's probably easiest to mitigate this by validating it.

How was the change tested?

Added a test to the existing client configuration tests. The existing tests still pass.

Change types

• • Bug fix (non-breaking change that fixes an issue)
• • New feature (non-breaking change that adds functionality)
• • Breaking change (backwards-incompatible fix or feature)

Checklist:

• • My code follows the code style of this project.
• • My change requires a change to Fauna documentation.
• • My change requires a change to the README, and I have updated it accordingly.

[ptpaterson]

🍬 LGTM