chore: cache SPI extension classes

[arjenzhou]

What is the purpose of the change

cache SPI extension classes

Brief changelog

add extension classes cache in SPI

Verifying this change

Checklist

• Make sure there is a GitHub_issue field for the change (usually
  before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should
  address just this issue, without pulling in other changes - one PR resolves one issue.
• Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency
  exist. If the new feature or significant change is committed, please remember to add sample in project.
• Add some description to documents if you are requesting to add a feature.
• GitHub Actions works fine on your own branch.

[codecov]

Codecov Report

Merging #27 (0c64409) into master (199c15c) will decrease coverage by 0.43%.
The diff coverage is 67.39%.

@@             Coverage Diff              @@
##             master      #27      +/-   ##
============================================
- Coverage     59.57%   59.13%   -0.44%     
- Complexity      146      150       +4     
============================================
  Files            33       34       +1     
  Lines           841      859      +18     
  Branches         55       58       +3     
============================================
+ Hits            501      508       +7     
- Misses          299      308       +9     
- Partials         41       43       +2     

Impacted Files	Coverage Δ
...c/main/java/de/xab/porter/common/util/Strings.java	100.00% <ø> (ø)
...de/xab/porter/transfer/jdbc/reader/JDBCReader.java	61.20% <0.00%> (ø)
...de/xab/porter/transfer/jdbc/writer/JDBCWriter.java	51.77% <0.00%> (ø)
.../porter/transfer/jdbc/writer/PostgreSQLWriter.java	0.00% <0.00%> (ø)
...porter/transfer/exception/ConnectionException.java	0.00% <0.00%> (ø)
...java/de/xab/porter/common/spi/ExtensionLoader.java	75.78% <76.56%> (+5.67%)	⬆️
core/src/main/java/de/xab/porter/core/Task.java	94.73% <100.00%> (ø)
...a/de/xab/porter/api/exception/PorterException.java	0.00% <0.00%> (-50.00%)	⬇️

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