Blob refactor: Add ability to import custom blob viewers

What does this MR do?

This is part of a bigger effort to refactor the repository browser.

In this MR we add the ability to import custom blob viewers. This is the 1st step towards migrating all HAML based viewers to VueJS.

In %14.1 we'll migrate _empty.html.haml, _text.html.haml and _download.html.haml (follow-ups to this MR).

Note, changes in this MR are behind the :refactor_blob_viewer feature flag.

Screenshots (strongly suggested)

No visual changes are expected.

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.

Related to #324314 (closed)