feat(eap): implement backend for "what tag values exist" RPC #18118

Job	Run time
pre-commit hooks	53s
Dataset Config Validation	26s
detect what files changed	4s
Linting - Rust	2m 37s
self-hosted-end-to-end	19m 48s
Build snuba CI image	9m 52s
mypy typing	1m 0s
Publish Snuba to DockerHub	0s
Front end tests for snuba admin	51s
Tests on multiple clickhouse versions (23.8.11.29.altinitystable)	10m 0s
sentry (0)	13m 38s
Tests and code coverage (test)	11m 45s
sentry (1)	13m 5s
Tests and code coverage (test_rust)	5m 4s
sentry (2)	10m 21s
Tests and code coverage (test_distributed)	18m 14s
sentry (3)	13m 20s
Tests and code coverage (test_distributed_migrations)	2m 30s
	2h 13m 28s

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