Set content disposition attachment to several endpoints

What does this MR do?

Sets the content disposition to attachment in the following endpoints:

• Profiles::KeysController#get_keys
• Projects::JobsController#raw
• SnippetsActions#raw
• Api::Files#get
• Api::Repositories#get
• Api::Snippets#get

What are the relevant issue numbers?

Refs https://gitlab.com/gitlab-org/gitlab-ce/issues/36103

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.