Allow configuring explicit driver class loader

[timja]

For more controlled classloading than was added in #881

I'm trying to use this to replace commons-dbcp2 in Jenkins, and neither of the classloading options that are available here work (docs: https://www.jenkins.io/doc/developer/plugin-development/dependencies-and-class-loading/)

Similar issue:
FasterXML/jackson-dataformat-xml#483 (comment)

I've tested this in Jenkins and it works.

[codecov]

Codecov Report

Attention: Patch coverage is 18.42105% with 31 lines in your changes missing coverage. Please review.

Project coverage is 70.26%. Comparing base (ed2da5f) to head (d78837f).
Report is 86 commits behind head on dev.

Files with missing lines	Patch %	Lines
.../java/com/zaxxer/hikari/util/DriverDataSource.java	0.00%	21 Missing ⚠️
src/main/java/com/zaxxer/hikari/HikariConfig.java	37.50%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #1825      +/-   ##
============================================
- Coverage     70.75%   70.26%   -0.50%     
- Complexity      575      576       +1     
============================================
  Files            26       26              
  Lines          2171     2189      +18     
  Branches        311      315       +4     
============================================
+ Hits           1536     1538       +2     
- Misses          484      498      +14     
- Partials        151      153       +2     

Files with missing lines	Coverage Δ
src/main/java/com/zaxxer/hikari/pool/PoolBase.java	72.16% <100.00%> (ø)
src/main/java/com/zaxxer/hikari/HikariConfig.java	75.95% <37.50%> (-1.58%)	⬇️
.../java/com/zaxxer/hikari/util/DriverDataSource.java	44.73% <0.00%> (-5.27%)	⬇️

... and 2 files with indirect coverage changes

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[brettwooldridge]

I don't like this change. First, any change to class signatures requires a major revision increment. Second, the usual pattern for adapting to a classloading framework is to switch the context classloader and then switch back.

var originalLoader = Thread.currentThread().getContextClassLoader();
try {
   Thread.currentThread().setContextClassLoader(myClassLoader);

   HikariConfig config = new HikariConfig();
   // setup
   return new HikariDataSource(config);
} finally {
   Thread.currentThread().setContextClassLoader(originalLoader);
}