feat: allow to use ca, pfx, key and cert as string

[alexander-akait]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

fixes #1412

webpack-server supports this options as Buffer and string, we need port this feature.

Motivation / Use-Case

Better DX with https

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #1542 into master will increase coverage by 0.9%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #1542     +/-   ##
=========================================
+ Coverage   74.34%   75.25%   +0.9%     
=========================================
  Files          10       10             
  Lines         690      687      -3     
=========================================
+ Hits          513      517      +4     
+ Misses        177      170      -7

Impacted Files	Coverage Δ
bin/webpack-dev-server.js	58.28% <ø> (+1.45%)	⬆️
lib/Server.js	81.77% <100%> (+0.5%)	⬆️

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