Customize connection in UrlResource getInputStream

Prior to this commit, the `AbstractFileResolvingResource` would
provide a default implementation for `customizeConnection` which only
sets the HTTP request method as "HEAD".
While this is consistent with its usage within that class (in
`exists()`, `contentLength()` or `lastModified()`), this is not opened
for general usage by sub-classes.

`UrlResource` is an example of that, where its `getInputStream()` method
does not call this customization method.

This not only prevents implementations from calling
`customizeConnection` in various cases, but it also misleads developers
as they might think that customizations will be applied automatically.

This commit ensures that `customizeConnection` is called in all relevant
places and that the configuration of the HTTP method is instead done in
each method as it is use case specific.

Fixes gh-28909

spring-core/spring-core.gradle

@@ -58,6 +58,7 @@ dependencies {
 	testImplementation("org.xmlunit:xmlunit-matchers")
 	testImplementation("io.projectreactor:reactor-test")
 	testImplementation("io.projectreactor.tools:blockhound")
+	testImplementation("com.squareup.okhttp3:mockwebserver")
 	testFixturesImplementation("com.google.code.findbugs:jsr305")
 	testFixturesImplementation("org.junit.platform:junit-platform-launcher")
 	testFixturesImplementation("org.junit.jupiter:junit-jupiter-api")