feat(eap): implement backend for "what tag values exist" RPC #18107

Job	Run time
pre-commit hooks	36s
detect what files changed	3s
mypy typing	1m 25s
Dataset Config Validation	51s
Linting - Rust	2m 35s
self-hosted-end-to-end	20m 56s
Build snuba CI image	10m 12s
Publish Snuba to DockerHub	0s
Front end tests for snuba admin	53s
Tests on multiple clickhouse versions (23.8.11.29.altinitystable)	9m 53s
sentry (0)	10m 50s
Tests and code coverage (test)	9m 51s
Tests and code coverage (test_rust)	4m 32s
sentry (1)	10m 24s
Tests and code coverage (test_distributed)	10m 10s
sentry (2)	10m 23s
Tests and code coverage (test_distributed_migrations)	2m 26s
sentry (3)	10m 18s
	1h 56m 18s

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