Serialize Perspective schema

[philippjfr]

Ensures that Perspective handles datetime and other dtypes correctly and also handles pandas (multi-)indexes and column pivots.

Fixes #2124

data = pd.DataFrame({
    "int": np.arange(100),
    "float": [i * 1.5 for i in range(100)],
    "bool": [True for i in range(100)],
    "date": [date.today() for i in range(100)],
    "datetime": [datetime.now() for i in range(100)],
    "string": [chr(64+i) for i in range(100)]
})


pn.pane.Perspective(
    data, sizing_mode='stretch_width', height=400
)

[codecov]

Codecov Report

Merging #2130 (0a69250) into master (089acd5) will decrease coverage by 0.37%.
The diff coverage is 10.09%.

@@            Coverage Diff             @@
##           master    #2130      +/-   ##
==========================================
- Coverage   84.19%   83.81%   -0.38%     
==========================================
  Files         182      182              
  Lines       21533    21637     +104     
==========================================
+ Hits        18129    18135       +6     
- Misses       3404     3502      +98     

Impacted Files	Coverage Δ
panel/pane/perspective.py	52.88% <9.25%> (-38.92%)	⬇️
panel/models/perspective.py	100.00% <100.00%> (ø)
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️

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