new feature: support if_not_exists in the Memory service

[carlsverre]

Feature Description

The Memory service should support the write_with_if_not_exists capability.

Problem and Solution

I use the Memory store for testing, and require the if_not_exists capability for correctness. This feature would allow the Memory store to be used for testing.

Additional Context

No response

Are you willing to contribute to the development of this feature?

• Yes, I am willing to contribute to the development of this feature.