feat: add handler

[rmontagnin]

Description

This PR adds back the file handler that has been deleted when migrating to support Go embed.

[titouanj]

I see that @easonlin404 is a member of swaggo and a maintainer of this repo. I'm pinging him to see if he can review this PR and maybe the related one in gin-swagger

[ubogdan]

@titouanj is this related to an issue ?

[titouanj]

@titouanj is this related to an issue ?

I guess this one is the closest to the actual problem: swaggo/gin-swagger#315

[rmontagnin]

@ubogdan This PR is needed in order to make it possible to properly support OpenAPI Spec v3 by bumping swag to v2. The handler you see in this PR is needed in order to make sure that the generated file can be served properly using the /swag/doc.json REST endpoint

[Utkarsh-dev14]

@ubogdan @easonlin404 Are we merging this PR?? So that we can migrate to v2

[hunshcn]

This helps drive the migration to v2.

@ubogdan

[ubogdan]

@RiccardoM I don't understand why you are going back to WebDAV when we have native embed.FS.