Add optional expires_at param to Users API endpoint Add SSH key

What does this MR do?

It exposes an additional (optional) parameter (expires_at) to the Users API/Add SSH key endpoint.

It also updates the Users API Add SSH key and Add SSH key for user by mentioning the new parameter.

Screenshots

The end results should look as follows.

Admin area

User Settings

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines

Availability and Testing

There is no risk to the quality/performance of the product and should only require additional integration tests.