[2.x] Customize dashboard domain and path using env vars

[mahansky]

This PR adds an option to customize dashboard domain and path (prefix) using environment variables, similar to Telescope or Horizon.

[codecov-io]

Codecov Report

Merging #671 (3ff362f) into 2.x (8baefdd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                2.x     #671   +/-   ##
=========================================
  Coverage     90.71%   90.71%           
  Complexity        2        2           
=========================================
  Files            60       60           
  Lines          1690     1691    +1     
=========================================
+ Hits           1533     1534    +1     
  Misses          157      157           

Impacted Files	Coverage Δ	Complexity Δ
src/WebSocketsServiceProvider.php	93.54% <100.00%> (+0.10%)	0.00 <0.00> (ø)

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