feat(eap): implement backend for "what tag values exist" RPC (#6304) #18119

Job	Run time
pre-commit hooks	35s
Linting - Rust	2m 37s
Dataset Config Validation	41s
detect what files changed	4s
self-hosted-end-to-end	20m 13s
Build snuba CI image	10m 19s
mypy typing	1m 4s
Publish Snuba to DockerHub	7m 53s
Front end tests for snuba admin	34s
Tests on multiple clickhouse versions (23.8.11.29.altinitystable)	9m 58s
sentry (0)	10m 43s
sentry (1)	10m 23s
Tests and code coverage (test)	9m 55s
sentry (2)	10m 33s
Tests and code coverage (test_rust)	4m 37s
sentry (3)	10m 40s
Tests and code coverage (test_distributed)	17m 25s
Tests and code coverage (test_distributed_migrations)	2m 21s
	2h 10m 35s

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