feat: support ftl.Map on injected resources

[worstell]

embed the FTL handle that needs mapping in a new mapper type, e.g.:

type DBMapper struct {
     ftl.DatabaseHandle[MyConfig]
}

then implement a map function on it:

func (DBMapper) Map(ctx context.Context, conn *sql.DB) (NewType, error) {
     return NewType{db}, nil
}

such that your new type implements this FTL interface:

type ResourceMapper[From, To any] interface {
     Handle[From]
     Map(ctx context.Context, f From) (To, error)
}

Then, you can plug your mapper into a verb signature via ftl.MappedHandle, e.g.:

//ftl:verb
func Insert(ctx context.Context, req Request, db ftl.MappedHandle[DBMapper, *sql.DB, NewType]) (Response, error) {
     ...
}

also fixes #2941

[worstell]

would prefer to leave these in the template for readability, but with the new MappedHandle type, we need to recurse back into this logic to get the string for the resource that will be mapped - so pulled it out into a Go function