Add get end client command

[massa-bot]

In GitLab by @AureliaDolo

Add a get_end command to the client to get the end timestamp of that episode.

And maybe in a countdown form like : 3 days and 7h17 remaining in episode X

[massa-bot]

In GitLab by @AureliaDolo

@damip @yvan-sraka @g-massa what do you think about that ?

[massa-bot]

In GitLab by @yvan-sraka

Could be useful, not convinced by the naming, what about get_end_timestamp_of_episode?

[massa-bot]

In GitLab by @damip

this is testnet-specific, I would vote for not polluting code with too many things that will have to be removed after the testnet ends

[massa-bot]

In GitLab by @AureliaDolo

That's not a lot a work and it answers a common question