Add prefix support for nested applications

[RHeynsZa]

As per aiohttp, nested applications are used to avoid monolithic files with all your endpoints.

I've been able to generate the swagger for every single sub_app, as well as the route app. The only issue is that the swagger has no idea of the prefix that is set on the aiohttp level

Example

main.py

app = web.Application()
s = SwaggerDocs(
    app,
    swagger_ui_settings=SwaggerUiSettings(path="/docs"),
    info=SwaggerInfo(
        title="Base API",
        version="1.0.0",
    ),
)
s.add_routes([web.get("/admin-health", handle_health, allow_head=False)])
admin = admin_factory.create_app()
app.add_subapp('/admin/', admin)

admin_factory.py

# 
app = web.Application()
s = SwaggerDocs(
    app,
    swagger_ui_settings=SwaggerUiSettings(path="/docs"),
    info=SwaggerInfo(
        title="Admin API",
        version="1.0.0",
    ),
)
s.add_routes([web.get("/admin-health", handle_health, allow_head=False)])
return app

Description

So this works, the only issue is that the admin api swagger has no clue of the prefix. ie
App health is -> <url>/health
Admin health is -> <url>/admin/admin-health

The swagger documentation wrongfully points to <url>/admin-health

Should be a pretty easy fix on SwaggerUiSettings