Add support for configuring Vega output

[philippjfr]

Should consider which other VegaEmbed options to expose or whether to simply let users provide a config dictionary.

Implements #2821

[codecov]

Codecov Report

Merging #2846 (dd196ae) into master (8dfb3e9) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head dd196ae differs from pull request most recent head 69f2c4c. Consider uploading reports for the commit 69f2c4c to get more accurate results

@@            Coverage Diff             @@
##           master    #2846      +/-   ##
==========================================
+ Coverage   83.04%   83.06%   +0.02%     
==========================================
  Files         190      188       -2     
  Lines       24554    24454     -100     
==========================================
- Hits        20390    20313      -77     
+ Misses       4164     4141      -23     

Impacted Files	Coverage Δ
panel/models/vega.py	94.11% <100.00%> (+0.78%)	⬆️
panel/pane/vega.py	82.92% <100.00%> (+0.13%)	⬆️
panel/auth.py	39.15% <0.00%> (-1.65%)	⬇️
panel/util.py	81.52% <0.00%> (-1.21%)	⬇️
panel/param.py	87.63% <0.00%> (-0.05%)	⬇️
panel/pane/markup.py	97.19% <0.00%> (-0.04%)	⬇️
panel/tests/test_param.py	99.87% <0.00%> (-0.01%)	⬇️
panel/config.py	58.95% <0.00%> (ø)
panel/widgets/ace.py	100.00% <0.00%> (ø)
... and 7 more

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