Allow Bulk Import to use relative GitLab URL

What does this MR do?

This MR does 2 things:

1. Updates BulkImports::Clients::HTTP to not reconstruct user entered URL and instead use it as is in order to allow relative gitlab URL to be possible to use. E.g. http://mywebsite.example/gitlab
2. Updates BulkImports::Clients::HTTP argument from uri to url for consistency. BulkImports::Clients::GraphQL client has url as parameter.

Mentions #330344 (closed)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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