Add a POST handler for render-diff

[yashrsharma44]

Added a POST handler for /render-diff
Fixes #321

Verification

Verified the endpoint using Postman -

• Request Body

{
	"query":"pyroscope.server.alloc_objects{}",
	"from":"now-1h",
	"until":"now",
	"max-nodes":1024,
	"left-params" : {
		"from":"1629120696",
		"until":"1629121946"
	},
	"right-params" : {
		"from":"now-30m",
		"until":"now"
	},
	"format":"json"
}

TODO -

• Tests for POST method

[codecov]

Codecov Report

Merging #338 (9c56b06) into main (615e268) will increase coverage by 0.55%.
The diff coverage is 52.24%.

@@            Coverage Diff             @@
##             main     #338      +/-   ##
==========================================
+ Coverage   53.93%   54.47%   +0.55%     
==========================================
  Files         102      102              
  Lines        4781     4834      +53     
==========================================
+ Hits         2578     2633      +55     
+ Misses       1964     1951      -13     
- Partials      239      250      +11     

Impacted Files	Coverage Δ
pkg/server/controller.go	31.64% <33.34%> (ø)
pkg/server/render.go	63.06% <53.13%> (+20.75%)	⬆️

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[kolesnikovae]

Looks good! I've left a couple of notes, please take a look :)

[petethepig]

Looks good, merging