[Feature Request] Support `TEMPORAL_API_KEY` environment variable for consistency with `temporal` CLI

[tomwheeler]

Is your feature request related to a problem? Please describe.

As per this internal Slack thread, I observed last night that the tcld command's environment variable used for API Key authentication (TEMPORAL_CLOUD_API_KEY) differs from the one used in the temporal CLI (TEMPORAL_API_KEY). This is confusing, adds friction for someone who uses both tools, and is a potential source of trouble for someone who doesn't realize they differ (since they will set one variable but not the other and then wonder why things didn't work as expected).

Describe the solution you'd like

For backwards compatibility, the TEMPORAL_CLOUD_API_KEY environment variable must continue to be supported. However, I would recommend making the TEMPORAL_API_KEY environment variable an alternate and preferred one that achieves the same effect. This will ensure that a user who sets the TEMPORAL_API_KEY environment variable will see consistent behavior in tcld, the temporal CLI, the SDKs when they support self-configuration in the future, the OMS reference application, and potentially other tools that rely on the variable set for the temporal CLI.

Additional context