fix(sentinel): load classpath rule files from packaged JARs

[Rajandeep56]

Describe what this PR does / why we need it

When Sentinel file rules are configured as a classpath resource (for example classpath:sentinel.json), the app works in an IDE but fails after java -jar with:
FileNotFoundException: class path resource [sentinel.json] cannot be resolved to absolute file path because it does not reside in the file system
FileDataSourceProperties.preCheck always converted the location with ResourceUtils.getFile(), which cannot resolve files packed inside a JAR.

Does this pull request fix one issue?

Fixes #462

Describe how you did it

If ResourceUtils.getFile() fails, load the Spring Resource as an InputStream and copy it to a temp file. FileRefreshableDataSource still receives a java.io.File, so filesystem files are unchanged.

Describe how to verify it

1. Configure a file datasource, e.g. spring.cloud.sentinel.datasource.ds1.file.file=classpath:flowrule.json.
2. Run from the IDE: rules load as before.
3. Package the app and run java -jar. Startup should succeed and rules should load from the classpath file in the JAR.
4. ./mvnw -pl spring-cloud-alibaba-starters/spring-cloud-alibaba-sentinel-datasource -am test

Special notes for reviews

JAR-packed rule files are copied to a temp file (deleteOnExit). They are not watched for live updates, which matches Sentinel’s assumption that files inside a JAR are immutable.

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