Support schemes for URL base definitions

[chrgernoe]

I have a use case where the Git server is accessed either via SSH and HTTPS, depending on the usage. For local development users use SSH to access the server and for CI, I have to use HTTPS because the CI user has no SSH key and often the infrastructure does not support SSH for security reasons.

Feature proposal:
Support scheme-specific url-base definitions for every remote (or maybe even an arbitrary url-base scheme) that can be selected during update/clone/... on the command line and via environment variable.

# For backward compatibility and also for repos that are for some reasons always accessible via a single scheme, the `url-base` key inside the TOML file should be supported as well.
[[remotes]]
name = "style0"
url-base = "https://..."

# Remote with different schemes
[[remotes]]
name = "style1"
url-base.ssh = "ssh://..."
url-base.https = "https://..."
url-base.file = "file://..."

# Invalid remote definition
[[remotes]]
name = "invalid"
url-base = "https://..."
url-base.ssh = "ssh://..."

[defaults]
remote = "syle0"
url-base-scheme = "ssh"

There shall be command line option for all relevant sub-commands to override the URL base scheme e.g. --url-base-scheme and the user should be able to override the default via an environment variable e.g. GIT_WS_URL_BASE_SCHEME. This makes it easy to switch the scheme without script changes e.g. in CI mode.