feat(spooler): Implement next fetch behavior in the spooler v2 #15402

Job	Run time
Test All Features (ubuntu-latest)	3m 22s
Test (macos-latest)	2m 46s
Test (windows-latest)	5m 56s
Lint	2m 54s
Lint Rust Default Features	1m 36s
Test Python	1m 7s
Setup build metadata	1s
Integration Tests	12m 5s
Build Relay Binary (relay, x86_64-unknown-linux-gnu)	7m 58s
Build Docker Image (relay)	29s
Sentry-Relay Integration Tests	10m 5s
Publish Relay to DockerHub	0s
self-hosted-end-to-end	13m 13s
Publish Relay to GCR	0s
Upload build artifacts to gocd	0s
	1h 1m 32s

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