[FEATURE] User endpoint auto-hash and validate

[bmikaili]

The User Endpoint is missing some functionality that I have implemented using a custom endpoint. E.g. when creating a User you should not need to calculate the Hash yourself. In additon some check if a user already exist should be in there, e.g. a unique username.

Description

Auto hash password upon creation of a user with the User component and also check if that username is already taken.

Related Issue

Closes #22

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• SwiftLint throws no warnings or errors.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.