Add TimeStamp options to XML, Appsetting and environment var

[304NotModified]

• Add internalLogIncludeTimestamp option to <nlog> (fixes includeTimestamp in configuration file not present #1320)
• add "nlog.internalLogIncludeTimestamp" <AppSettings>
• add NLOG_INTERNAL_INCLUDE_TIMESTAMP environment setting

Docs: https://github.com/NLog/NLog/wiki/Internal-Logging

[codecov-io]

Current coverage is 75.14%

Merging #1323 into master will decrease coverage by -0.07% as of 45935bc

@@            master   #1323   diff @@
======================================
  Files          267     267       
  Stmts        15518   15519     +1
  Branches      1630    1631     +1
  Methods          0       0       
======================================
- Hit          11672   11662    -10
- Partial        383     384     +1
- Missed        3463    3473    +10

Review entire Coverage Diff as of 45935bc

---

Uncovered Suggestions

1. +0.09% via ...gingConfiguration.cs#374...387
2. +0.09% via ...gingConfiguration.cs#340...353
3. +0.09% via ...argets/FileTarget.cs#1713...1725
4. See 7 more...

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[AuthorProxy]

update xsd file #1977