(Re-)Use 'consumer_uuid' request attribute in negotiator

Problem/Motivation

If \Drupal\consumers\Negotiator::negotiateFromRequest() is called multiple times for the same request (and if the request actually has a consumer ID header) each call will incur a database query to load the respective consumer.

Steps to reproduce

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Proposed resolution

Since we already store the consumer UUID as a request attribute, we might as well check if the given request already has that set and then early return without (re-)loading the consumer.

Remaining tasks

User interface changes

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API changes

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Data model changes

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