Add generic API credential fallbacks to dispatch CLI

[Marenz]

Adds FREQUENZ_API_KEY / FREQUENZ_API_SECRET fallbacks to dispatch-cli, following frequenz-io/rnd#187.

Explicit options take precedence over the Dispatch-specific environment pair, which takes precedence over the generic pair. Keys and signing secrets are never mixed across sources.

[llucax]

I would hold on this one too:

1. We need to make sure we agree on whatever name you want to propose, to avoid having to change everything again (including CLI args, constructor parameters and env variables, I would align them ideally, so either --api-key/api_key=/_API_KEYor--key/key=/_KEY`, but not a mix of both).
2. I would do the change top-down (i.e. change client-base first), so it gets more visibility and all projects get updated, otherwise it only adds more confusion if some projects use one and others the other
3. Unrelated, but I would consider removing the env var support, at least for the client library (we can keep it as a CLI-only feature, it makes more sense there). Even when "automatic auth" can be convenient, I guess making auth explicit in code is not a crazy requirement.

[Marenz]

Unrelated, but I would consider removing the env var support, at least for the client library (we can keep it as a CLI-only feature, it makes more sense there). Even when "automatic auth" can be convenient, I guess making auth explicit in code is not a crazy requirement.

It is a CLI only feature, the client/library itself knows nothing about env vars :)

[Marenz]

@cwasicki This includes the fix for the epoch timestamp bug you discovered in frequenz-io/frequenz-actor-electricity-trading#570 — passing a numpy.datetime64 (from TimeSeries.index) to create(start_time=...) silently produced epoch dispatches. The fix raises ValueError immediately for anything that isn't a datetime or "NOW", and the corresponding fix in the actor should convert the index timestamps to proper datetime objects before constructing BatteryPowerScheduleEntry.

[cwasicki]

The start time fix LGTM, thank you!