set default extPlugin path to relative path.

[initialdmg]

Current default value of extPlugin path /ext-lib/ is not consistent with described in deployment document，which resulting custom plugin jars auto loading failed. On Windows, bootstrap binary package's startup context is bin directory, ext plugin relative path is not correct too.

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.

[yu199195]

Why can't use '/ ' at the beginning?

[initialdmg]

The absolute path of /ext-lib/ is not in the deployment directory. On Linux, it's just pointing to the /ext-lib folder. On Windows it is in the root of the deployment driver. A log point added to confirm this. We can find this in test case too:

shenyu/shenyu-web/src/test/java/org/apache/shenyu/web/loader/ShenyuPluginPathBuilderTest.java

Lines 42 to 49 in 1f5b5cd

	* test for plugin-ext.
	* -Dplugin-ext=D:\testUrl
	*/
	@Test
	public void testGetPluginPathByPluginExt() {
	System.setProperty("plugin-ext", "/testUrl");
	File jarFile = ShenyuPluginPathBuilder.getPluginFile("");
	assertNotNull(jarFile);