Issue #3606290: Rename the node feature to External interaction

Frank Mablyrequested to merge
issue/orchestra-3606290:3606290-interaction-label into 1.x

Follow-up to #3606290. The interaction node feature is the generic external-party side of a workflow (the mirror of orchestra_inbox), not a checkout or customer concept, so its "Customer interaction" label drifted back into booking vocabulary.

Rename the label and the CustomerInteraction class to External interaction / ExternalInteraction. The plugin id (interaction) and the node config key are unchanged, so there is no config migration. Also neutralizes a couple of customer-specific doc phrasings (README, interface docblocks).

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