Support custom properties file formats in @TestPropertySource

Spring Framework 4.3 introduced the `PropertySourceFactory` SPI for use
with `@PropertySource` on `@Configuration` classes; however, prior to
this commit there was no mechanism to support custom properties file
formats in `@TestPropertySource` for integration tests.

This commit introduces support for configuring a custom
`PropertySourceFactory` via a new `factory` attribute in
`@TestPropertySource` in order to support custom file formats such as
JSON, YAML, etc.

For example, if you create a YamlPropertySourceFactory, you can use it
in integration tests as follows.

@SpringJUnitConfig
@TestPropertySource(locations = "/test.yaml", factory = YamlPropertySourceFactory.class)
class MyTestClass { /* ... /* }

If a custom factory is not specified, traditional `*.properties` and
`*.xml` based `java.util.Properties` file formats are supported, which
was the existing behavior.

Closes gh-30981

spring-test/spring-test.gradle

@@ -85,6 +85,7 @@ dependencies {
 	testRuntimeOnly("org.junit.vintage:junit-vintage-engine") {
 		exclude group: "junit", module: "junit"
 	}
+	testRuntimeOnly("org.yaml:snakeyaml")
 }
 
 // Prevent xml-apis from being used so that the corresponding XML APIs from