Introduce sharding rules to MongoDB collections

[sejongk]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #673

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Attention: 235 lines in your changes are missing coverage. Please review.

Comparison is base (8a33500) 48.81% compared to head (94dffbc) 49.26%.
Report is 8 commits behind head on main.

Files	Patch %	Lines
server/backend/database/memory/database.go	51.09%	63 Missing and 4 partials ⚠️
server/backend/database/mongo/client.go	61.11%	63 Missing ⚠️
server/rpc/yorkie_server.go	79.77%	17 Missing and 1 partial ⚠️
client/client.go	0.00%	16 Missing ⚠️
server/backend/sync/memory/pubsub.go	42.30%	15 Missing ⚠️
server/rpc/admin_server.go	44.44%	15 Missing ⚠️
server/backend/database/mongo/registry.go	70.83%	10 Missing and 4 partials ⚠️
server/backend/database/client_info.go	69.76%	13 Missing ⚠️
api/types/resource_ref_key.go	57.14%	6 Missing ⚠️
server/backend/housekeeping/housekeeping.go	0.00%	4 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #689      +/-   ##
==========================================
+ Coverage   48.81%   49.26%   +0.44%     
==========================================
  Files          69       70       +1     
  Lines       10127    10300     +173     
==========================================
+ Hits         4944     5074     +130     
- Misses       4657     4705      +48     
+ Partials      526      521       -5     

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