pageserver: fixes + test updates for sharding (#6186)

This is a precursor to:
- https://github.com/neondatabase/neon/pull/6185

While that PR contains big changes to neon_local and attachment_service,
this PR contains a few unrelated standalone changes generated while
working on that branch:
- Fix restarting a pageserver when it contains multiple shards for the
same tenant
- When using location_config api to attach a tenant, create its
timelines dir
- Update test paths where generations were previously optional to make
them always-on: this avoids tests having to spuriously assert that
attachment_service is not None in order to make the linter happy.
- Add a TenantShardId python implementation for subsequent use in test
helpers that will be made shard-aware
- Teach scrubber to read across shards when checking for layer
existence: this is a refactor to track the list of existent layers at
tenant-level rather than locally to each timeline. This is a precursor
to testing shard splitting.

test_runner/performance/test_bulk_insert.py

@@ -61,7 +61,6 @@ def measure_recovery_time(env: NeonCompare):
     # of view, but the same as far as the safekeeper/WAL is concerned.  To work around that,
     # we will explicitly create the tenant in the same generation that it was previously
     # attached in.
-    assert env.env.attachment_service is not None
     attach_status = env.env.attachment_service.inspect(tenant_id=env.tenant)
     assert attach_status is not None
     (attach_gen, _) = attach_status