feat(eap): implement backend for "what tag values exist" RPC #18115

Job	Run time
detect what files changed	3s
pre-commit hooks	39s
Linting - Rust	2m 34s
mypy typing	1m 4s
Dataset Config Validation	35s
self-hosted-end-to-end	21m 46s
Build snuba CI image	9m 27s
Publish Snuba to DockerHub	0s
Front end tests for snuba admin	52s
Tests on multiple clickhouse versions (23.8.11.29.altinitystable)	9m 56s
Tests and code coverage (test)	10m 38s
sentry (0)	10m 43s
Tests and code coverage (test_rust)	4m 44s
sentry (1)	10m 35s
Tests and code coverage (test_distributed)	17m 28s
sentry (2)	10m 26s
Tests and code coverage (test_distributed_migrations)	2m 26s
sentry (3)	11m 1s
	2h 4m 57s

Provide feedback