Add service for uploading files in the Content Editor

What does this MR do?

NOTICE: This MR is part of a larger effort to allow inserting images in the Content Editor. To get a full picture and understand how this change fits in this goal, check: !63939 (merged)

This MR implements a module that:

• Uploads a file by sending a post request with a form data payload that contains a file attachment.
• It takes the link property of the object returned by the upload request, which contains a Markdown link definition, and renders it using the GitLab Markdown API.
• It extracts the canonicalSrc and src properties generated by the Markdown API and returns them.

Other MRs

• Insert images in the Content Editor
• Provide renderMarkdown and uploadsPath to Image extension
• Display Content Editor errors
• Upload file service
• Image node view wrapper
• Add file upload support to image extension
• Insert image toolbar button

Screenshots or Screencasts (strongly suggested)

This MR does not introduce user-facing changes

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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
• Security reports checked/validated by a reviewer from the AppSec team

Related to #328641 (closed)