Support Multilevel base path mapping : Update to API Gateway V2

**Issue**
When using custom domain names, if the _url_prefix_ has `/` in it, the deployment fails with the error

```
An error occurred (BadRequestException) when calling the CreateBasePathMapping
operation: API Gateway V1 doesn't support the slash character (/) in base
path mappings. To create a multi-level base path mapping, use API Gateway V2.
```
**Request**
It would make it easier to isolate and distribute logic across multiple functions if Chalice supports multi-level base path mapping by updating to API Gateway V2

Start by locating the deployment path that calls CreateBasePathMapping and review the API Gateway V2 base path mapping requirements for slash-containing url_prefix values. Done means custom-domain deployments with multilevel base paths succeed instead of failing with the API Gateway V1 error.

Written by the indexing model from the issue text.