Add method to compute the count for every lineage

[Taepper]

SILO has the Lineage query, which computes the count for a lineage (optionally) including its descendants:
https://github.com/GenSpectrum/LAPIS-SILO/blob/main/documentation/query_documentation.md#lineage-column-string-value-[…]g-includesublineages-boolean

Loculus wants to receive this count for every lineage in the LineageDefinitions. This should be supported by silo