support count(*) on sharded table

[dk-lockdown]

Is your feature request related to a problem? Please describe.
if topology is like that:

db: drug_0, table: drug_resource_0, drug_resource_1, drug_resource_2, drug_resource_3, drug_resource_4
db: drug_1, table: drug_resource_5, drug_resource_6, drug_resource_7, drug_resource_8, drug_resource_9

drug_resource table contains columns: id, name, code, purchase_price, etc.

when query select count(*) from drug_resource where id > 15 and id <18, should rewrite the sql, query on every shard, then sum each result.

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