feat(eap): implement backend for "what tag values exist" RPC #18117

Job	Run time
detect what files changed	6s
pre-commit hooks	42s
Dataset Config Validation	29s
Linting - Rust	2m 38s
self-hosted-end-to-end	20m 4s
Build snuba CI image	9m 20s
mypy typing	1m 7s
Publish Snuba to DockerHub	0s
Front end tests for snuba admin	52s
Tests on multiple clickhouse versions (23.8.11.29.altinitystable)	14m 14s
Tests and code coverage (test)	10m 7s
Tests and code coverage (test_rust)	10m 17s
sentry (0)	12m 55s
sentry (1)	11m 49s
Tests and code coverage (test_distributed)	17m 41s
sentry (2)	12m 39s
Tests and code coverage (test_distributed_migrations)	7m 44s
sentry (3)	14m 53s
	2h 27m 37s

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