Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/348874

Instead of using inheritance to share code among the project creation
classes, I refactor it to use composition. This away the code is a bit
more extensible and has more independent parts (less coupling).

`Import::GitlabProjects::CreateProjectFromRemoteFileService` and
`Import::GitlabProjects::CreateProjectFromUploadedFileService` was
replaced by a single entry point
`Import::GitlabProjects::CreateProjectService`.

`Import::GitlabProjects::CreateProjectService` is responsible to create
a project when the user parameters are valid. Since the user can use
different strategies to create/import a project, the requirements for
each strategy is implemented in different
`Import::GitlabProjects::Strategies::*` classes, this way the strategy
can be changed by dependency injection.

A `Import::GitlabProjects::Strategies::*` class must implement:
- initialize(current_user:, params:)
- valid? - validate the user parameters
- errors - when invalid, a ActiveModel::Errors object
- project_params - A hash with the specific strategy extra parameters to
  create the project

Also, because importing from a generic object storage and from AWS S3
have similar validations to be performed, the
`Import::GitlabProjects::Strategies::RemoteFileValidator` was created to
validate the remote file `content_length` and `content_type`.

MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77259