Update snippets base service and callers

Why

We want to use reason instead of http_status in accordance with our development guidelines and Rubocop rules.

Also, we want to start returning symbol reasons and have any http layer callers interpret that as needed into http status codes. This MR introduces a new class to handle that and uses it in the api layer.

What

• replace http_status with reason
• since this is a base class, identify inheriting classes that do not override the affected method(Snippets::CreateService, and Snippets::UpdateService)
• find callers of inheritors
• update them to use reason instead of http_status when interacting with the service responses.
• add map class to interpret reasons to http status codes

Related to #521438 (closed)