Add spec for cases where URI.join does not work as expected

If an environment slug is predictable given only the environment
name, we can use the environment slug earlier in the CI variable
evaluation process as we don't have to wait for the environment
record itself to be persisted.

This enables terminals for group and project level clusters.
Previously there was no way to determine which project (and
therefore kubernetes namespace) to connect to, moving this
logic onto Environment means the assoicated project can be
used to look up the correct namespace.

available? is much cheaper than deployment_platform

If environment is not available, then no point running the more
expensive `prometheus_adapter`.

As this method does have `deployment_platform(environment: self.name)`,
which is what the form EE needs. Allows us to remove two overrides in
EE.

Renamed UrlValidator to AddressableUrlValidator to avoid 'url:' naming collision with ActiveModel::Validations::UrlValidator in 'validates' statement.
Make use of the options attribute of the parent class ActiveModel::EachValidator.
Add more options: allow_nil, allow_blank, message.
Renamed 'protocols' option to 'schemes' to match the option naming from UrlValidator.

Adds support for start and end parameters in the #additional_metrics
endpoint of the EnvironmentsController. start and end are meant to be
unix timestamps, per the Prometheus API (as the consumer of this
endpoint will eventually be transitioned to a prometheus endpoint).
This functionality is behind the :metrics_time_window feature flag
for development.