[alerting][actions] change isESOUsingEphemeralEncryptionKey determination

[pmuellr]

See issue #79943 (comment) .

If we end up implementing the proposed behavior in that issue of making the ESO plugin optional, and having it disable itself if the encryption key isn't set, we'll need to change the logic in the actions and alerts plugins to accomodate.

Presumably we'll change the internal properties we're using isESOUsingEphemeralEncryptionKey to reflect the new processing, perhaps something like isESOavailable or such.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[mikecote]

Some potential challenges in regards to migrations: #79943 (comment)

[azasypkin]

Hey @elastic/kibana-alerting-services,

Can you please confirm that you're still targeting 7.12 for this work? Once you merge your part we'll remove usingEphemeralEncryptionKey from the EncryptedSavedObjects plugin contract in #81511.

Thanks!

[mikecote]

@azasypkin yup, I can confirm this issue made it part of our 7.12 release planning. Once this issue closes, you'll be good to go on your end 👍

[YulNaumenko]

Closing in favor of the PR which replaces isESOUsingEphemeralEncryptionKey with isESOCanEncrypt based on the new ESO property canEncrypt.
Opened related issue which will cover the second part of the story about handling migrations.