Using a custom S3FS hostname duplicates its http(s) protocol

Problem/Motivation

When S3FS is configured to use a custom hostname, the protocol is duplicated when trying to upload a file (i.e. https://https://custom.host:port).

Steps to reproduce

On the S3FS settings page (/admin/config/media/s3fs)
- check Use a Custom Host, then enter a custom host name in the field below. Note that the description of the Hostname field instructs users to enter the host name including the protocol ("https://objects.example.com") since #3207028: Decouple custom endpoint http/https selection from 'use_https' config →
- check Use path-style endpoint
- try to upload a file using the S3 storage

Proposed resolution

Don't prepend a protocol to the endpoint when using a custom host name.

Remaining tasks

Review.

User interface changes

Nope.

API changes

Nope.

Data model changes

Nope.