Support mills timestamp format

[marandaneto]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

Support mills timestamp format.
eg 1581410911.988
which is {seconds}.{mills}

💡 Motivation and Context

JS sends mills timestamp format and we need to support hybrid SDKs.

💚 How did you test it?

unit test.

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• All tests passing

🔮 Next steps

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@29fdaaf). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master     #263   +/-   ##
=========================================
  Coverage          ?   57.13%           
  Complexity        ?      576           
=========================================
  Files             ?       73           
  Lines             ?     2767           
  Branches          ?      235           
=========================================
  Hits              ?     1581           
  Misses            ?     1068           
  Partials          ?      118

Impacted Files	Coverage Δ	Complexity Δ
...y-core/src/main/java/io/sentry/core/DateUtils.java	55.55% <0%> (ø)	4 <0> (?)

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