Add UNIX epoch <-> TIMESTAMP conversion functions

[alancai98]

Adds functions to convert from UNIX epoch to TIMESTAMP and TIMESTAMP to UNIX epoch. Largely follows MySQL's variants: FROM_UNIXTIME and UNIX_TIMESTAMP.

Still might need discussion on:

• Better names for the functions. I just followed MySQL's convention given that I couldn't find similar functions in other DBMS and SQL-92 standard. Perhaps UNIX_TO_TIMESTAMP and TIMSTAMP_TO_UNIX would be better?
• Supporting a format argument when converting UNIX epoch -> TIMESTAMP. This is an additional argument in MySQL's version, but this may be better served as its own function.
• Datatype input for UNIX epoch -> TIMESTAMP currently is just a numeric value. Do we want to allow for strings (e.g. from_unixtime("1234567890"))?
• Datatype to support for UNIX epochs? I largely followed MySQL's FROM_UNIXTIME which uses decimal depending on if the timestamp had fractional seconds, integer otherwise.
• Some edge cases, namely: 1. What should we do when given a UNIX epoch to convert less than 0? 2. What do we do when given a timestamp before '1970-01-01 00:00:01.000000' UTC? (currently return 0)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[alancai98]

New commit removes the negative unix epoch / timestamp before the last epoch restriction and updates tests + docs accordingly. Also adds UNIX_TIMESTAMP and FROM_UNIXTIME entries to BuiltinFunctions.md.

[codecov-io]

Codecov Report

Merging #330 (f9ff9d5) into master (2264117) will increase coverage by 0.03%.
The diff coverage is 95.23%.

@@             Coverage Diff              @@
##             master     #330      +/-   ##
============================================
+ Coverage     79.58%   79.61%   +0.03%     
- Complexity     1190     1197       +7     
============================================
  Files           157      159       +2     
  Lines          9371     9391      +20     
  Branches       1526     1527       +1     
============================================
+ Hits           7458     7477      +19     
- Misses         1472     1473       +1     
  Partials        441      441              

Flag	Coverage Δ	Complexity Δ
CLI	0.00% <ø> (ø)	0.00 <ø> (ø)
EXAMPLES	0.00% <ø> (ø)	0.00 <ø> (ø)
LANG	85.27% <95.23%> (+0.02%)	0.00 <7.00> (ø)
PTS	100.00% <ø> (ø)	0.00 <ø> (ø)
TEST_SCRIPT	79.68% <ø> (ø)	0.00 <ø> (ø)

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Impacted Files	Coverage Δ	Complexity Δ
...g/src/org/partiql/lang/eval/ExprValueExtensions.kt	81.13% <50.00%> (-0.40%)	0.00 <0.00> (ø)
...org/partiql/lang/eval/builtins/BuiltinFunctions.kt	95.12% <100.00%> (+0.25%)	0.00 <0.00> (ø)
...partiql/lang/eval/builtins/FromUnixTimeFunction.kt	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...artiql/lang/eval/builtins/UnixTimestampFunction.kt	100.00% <100.00%> (ø)	5.00 <5.00> (?)

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