fix: add support for java.sql.Timestamp GraphQLScalar coercing

[igdianov]

This PR adds support for parsing and serializing Timestamp scalar value, i.e.

• Epoch time in milliseconds
• ISO Instant
• ISO Offset Date/Time
• ISO Zoned Date/Time
• ISO Local Date/Time
• ISO Local Date

The output of serialize is a string in ISO 8086 Date/Time in UTC time zone, i.e. "2019-08-05T07:15:07Z"

The output of parseValue and parseLiteral is java.sql Timestamp value in UTC time zone.

[codecov]

Codecov Report

Merging #283 into master will increase coverage by 0.50%.
The diff coverage is 94.64%.

@@             Coverage Diff              @@
##             master     #283      +/-   ##
============================================
+ Coverage     74.08%   74.58%   +0.50%     
+ Complexity     1072     1071       -1     
============================================
  Files            72       72              
  Lines          4765     4801      +36     
  Branches        681      683       +2     
============================================
+ Hits           3530     3581      +51     
+ Misses          929      912      -17     
- Partials        306      308       +2     

Impacted Files	Coverage Δ	Complexity Δ
...ventures/graphql/jpa/query/schema/JavaScalars.java	57.80% <94.64%> (+10.38%)	6.00 <0.00> (ø)
.../graphql/jpa/query/introspection/Constructors.java	78.00% <0.00%> (-4.00%)	14.00% <0.00%> (-1.00%)

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