fix(db-postgres): stabilize read replicas support

[r1tsuu]

• Fixes Using read replicas: Error: Cannot use 'in' operator to search for '_rels' in undefined #13011 and 404 Error - Media Upload + Read Replica #13588
• Adds postgres-replicas configuration to our docker-compose.yml.
• Enables CI tests with postgres-read-replicas database adapter option.
• Adds dedicated test suite test/database/pg-replica/int.spec.ts for the readReplicas property

Problem

When readReplicas is configured, reads that happen as part of writes (e.g. reading back the row after an INSERT) were incorrectly routed to the replica instead of the primary, causing NotFound errors due to replication lag.

Solution

• getPrimaryDb - used in upsertRow, updateOne, updateMany to always use the primary DB for internal reads in write operations.
• readReplicasAfterWriteInterval - time-based read-after-write consistency. Before, if you did for example payload.create and immediately payload.find after - the new document would not show due to replication lag. Defaults to 2000 (ms) but can be configured.

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[github-actions]

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[JarrodMFlesch]

Sasha and I chatted about the confusing db variables being created and Sasha landed on passing the result of getTransaction directly into getPrimaryDB. This change made it easier to read and I think less error prone going forward. Now there is only 1 db var in each file.

[github-actions]

🚀 This is included in version v3.81.0