Enable Project Import to import from remote object storage [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Currently, the Infrastructure SRE team is frequently asked by Customer Support to run manual exports or imports because they fail due to an enforced size-limit on GitLab.com on http transfers via web and API. To avoid that, this MR adds a new endpoint to import a project from a remote object storage, this way we won't to go through the upload limits.

Current Approach

sequenceDiagram

    User->>+Workhorse: POST /project/import (project.tar.gz)
    note over User, Workhorse: Because the file needs to be uploaded, <br>The file size is limitted by the <br> uploads limits in the ".com"
    Workhorse->>+ObjectStorage: Download project.tar.gz
    ObjectStorage->>-Workhorse: OK
    Workhorse->>+Rails: POST /project/import
    Rails-->>-User: OK
    
    ImporterWorker->>+ObjectStorage: Fetch
    activate ImporterWorker
    ObjectStorage-->>-ImporterWorker: project.tar.gz
    ImporterWorker-->>ImporterWorker: process file
    deactivate ImporterWorker

New approach

sequenceDiagram

    User->>+Rails: POST /project/remote-import (URL for project.tar.gz)
    Rails-->>-User: OK
    note over User,Rails: Instead of passing the file, the user will now pass the URL for the file.<br>This will be saved in the import_export_upload table<br>to be used later in the ImporterWorker to download the file <br>and  process the import.
    
    ImporterWorker->>+RemoteObjectStorage: Fetch
    activate ImporterWorker
    RemoteObjectStorage-->>-ImporterWorker: project.tar.gz
    ImporterWorker-->>ImporterWorker: process file
    deactivate ImporterWorker

Related to: #282503 (closed)

Database migrations

rails db:migrate:up VERSION=20210419203017

$ rails db:migrate:up VERSION=20210419203017
== 20210419203017 AddRemoteImportUrlToImportExportUpload: migrating ===========
-- add_column(:import_export_uploads, :remote_import_url, :text)
   -> 0.0021s
== 20210419203017 AddRemoteImportUrlToImportExportUpload: migrated (0.0021s) ==

rails db:migrate:down VERSION=20210419203017

$ rails db:migrate:down VERSION=20210419203017
== 20210419203017 AddRemoteImportUrlToImportExportUpload: reverting ===========
-- remove_column(:import_export_uploads, :remote_import_url)
   -> 0.0082s
== 20210419203017 AddRemoteImportUrlToImportExportUpload: reverted (0.0083s) ==

rails db:migrate:up VERSION=20210419203018

$ rails db:migrate:up VERSION=20210419203018
== 20210419203018 AddRemoteTextLimitToImportUrlInImportExportUpload: migrating
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE import_export_uploads\nADD CONSTRAINT check_58f0d37481\nCHECK ( char_length(remote_import_url) <= 512 )\nNOT VALID;\n")
   -> 0.0040s
-- current_schema()
   -> 0.0002s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- execute("ALTER TABLE import_export_uploads VALIDATE CONSTRAINT check_58f0d37481;")
   -> 0.0012s
-- execute("RESET ALL")
   -> 0.0005s
== 20210419203018 AddRemoteTextLimitToImportUrlInImportExportUpload: migrated (0.0162s)

rails db:migrate:down VERSION=20210419203018

$ rails db:migrate:down VERSION=20210419203018
== 20210419203018 AddRemoteTextLimitToImportUrlInImportExportUpload: reverting
-- execute("ALTER TABLE import_export_uploads\nDROP CONSTRAINT IF EXISTS check_58f0d37481\n")
   -> 0.0011s
== 20210419203018 AddRemoteTextLimitToImportUrlInImportExportUpload: reverted (0.0086s)

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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