feat: implement query endpoints

[hallazzang]

Description

closes:

Tasks

• /stakings/{farmer}
• /total_stakings/{staking_coin_denom}
• /rewards/{farmer}
• Write tests

---

Before we can merge this PR, please make sure that all the following items have been
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• Appropriate labels applied
• Targeted PR against correct branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #114 (736d3e8) into master (600c2cc) will increase coverage by 0.62%.
The diff coverage is 85.05%.

@@            Coverage Diff             @@
##           master     #114      +/-   ##
==========================================
+ Coverage   65.39%   66.01%   +0.62%     
==========================================
  Files          24       24              
  Lines        2040     2107      +67     
==========================================
+ Hits         1334     1391      +57     
- Misses        575      580       +5     
- Partials      131      136       +5     

Impacted Files	Coverage Δ
x/farming/types/keys.go	0.00% <0.00%> (ø)
x/farming/keeper/reward.go	87.57% <66.66%> (ø)
x/farming/keeper/grpc_query.go	86.86% <85.50%> (-1.72%)	⬇️
x/farming/keeper/staking.go	90.47% <92.85%> (ø)

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[dongsam]

LGTM, If you set the epic issue to closes: in PR, all the tasks of the epic are not finished yet and may be closed. Therefore, I just modified the closes:.