Add ability to define admin dashboard plugins

[philippjfr]

Allows defining admin dashboard plugins by registering tuples of a title and function that returns a Panel object that will be rendered in the, e.g. you can do:

panel serve myapp.py --setup init.py

and then in init.py:

import panel as pn

def plugin():
    return '# My custom admin dashboard plugin'

pn.config.admin_plugins.append(('Custom Plugin', plugin))

[codecov]

Codecov Report

Merging #3668 (6dad6bc) into master (de3f978) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #3668      +/-   ##
==========================================
- Coverage   82.21%   82.21%   -0.01%     
==========================================
  Files         205      205              
  Lines       27934    27936       +2     
==========================================
+ Hits        22966    22967       +1     
- Misses       4968     4969       +1     

Impacted Files	Coverage Δ
panel/io/admin.py	0.00% <0.00%> (ø)
panel/config.py	60.00% <100.00%> (+0.11%)	⬆️

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