Add support for multiple files via pattern in MultiResourceItemReaderBuilder

[benelog]

It resolves #5056

The method name intentionally uses filesPattern instead of resourcesPattern, since in most cases the target resources are file-based. Internally, MultiResourceItemReaderBuilder.build() resolves resources using PathMatchingResourcePatternResolver.getResources(String) with a "file:" prefix, which aligns naturally with this naming choice.

I also attempted to add a test case to cover the catch block in the following code segment:

try {
	Resource[] resources = patternResolver.getResources("file:" + this.filesPattern);
	reader.setResources(resources);
} catch (IOException e) {
	throw new IllegalArgumentException(
			"Unable to initialize resources by the pattern " + this.filesPattern, e);
}

However, I could not find a case that would actually trigger this block.
When no files are found for the specified pattern, PathMatchingResourcePatternResolver simply returns an empty Resource[] rather than throwing an exception.

I considered making the builder throw an exception when the resolved Resource[] has zero length, but decided it would be more appropriate to delegate such behavior to MultiResourceItemReader or its internal delegate. For example, FlatFileItemReader determines whether to throw an exception for missing files based on its strict property.