Improve gin support (interface, error, middlewares)

[zzh8829]

1. Use gin.IRouter interface instead of *gin.Engine
   this will allow us to support nested router mounting using engine/routergroup/other routers
   fixes Support RouterGroup when registering handlers with gin #507
2. switch custom middleware code to gin middleware
   the custom middleware code doesn't support context abort or c.Next() in normal gin middleware, this also fixes gin: abort request in case middleware function called abort #494 gin: included middleware cannot abort request #485
3. add errors from validation middleware to gin context
   huge fix for debugging authentication func. before this change the nested e.Errors[] is completely hidden not surfaced anywhere.

[ericvolp12]

Hey, this is neat but it ended throwing off how we were constructing the gin engine, looks like now the pattern needs to be:

// Register the business logic handlers
apiRoutes := api.RegisterHandlers(r, apiServer).(*gin.RouterGroup)

r.Use(apiRoutes.Handlers...)

Instead of what was previously:

// Register the business logic handlers
r := api.RegisterHandlers(r, apiServer)

I'm gonna submit a MR to update the Gin example since this new patch breaks the example.

[jxsl13]

returning concrete types should be preferred over returning interfaces as concrete types that implement an interface may provide more functionality than the interface.
on the other hand requiring interfaces to be passed as parameters makes parameters more versatile.

[deepmap-marcinr]

huh, yeah. Agreed, returning concrete types is better. I missed that. I wonder whether to roll back this change. I hate breaking existing code like this.

[ericvolp12]

We could update it to return a gin.RouterGroup instead of the IRouter but I'm not 100% sure if there are any use cases in which we'd want an IRouter over the RouterGroup. AFAIK the IRouter interface was almost pulled from Gin a while ago and it has no use other than as a wrapper interface around a RouterGroup.

[jxsl13]

release a new minor version and retract the last one maybe in go.mod?

https://golangtutorial.dev/tips/retract-go-module-versions/