add support for XML-based changelogs

this provides the necessary XSD. note that the XSD has to be published
at
http://www.liquibase.org/xml/ns/opensearch/liquibase-opensearch-1.0.xsd
for the reference to be valid for consumers. this seems to have been
done for other plugins (e.g. mongodb) but is - presumably? - a manual
process.

src/main/resources/www.liquibase.org/xml/ns/opensearch/liquibase-opensearch-1.0.xsd

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
+            targetNamespace="http://www.liquibase.org/xml/ns/opensearch"
+            xmlns="http://www.liquibase.org/xml/ns/opensearch"
+            elementFormDefault="qualified">
+
+    <xsd:simpleType name="httpMethods" final="restriction">
+        <xsd:restriction base="xsd:string">
+            <!--
+             as per https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods
+
+             the following methods have been intentionally skipped as they cannot be used to modify data in OpenSearch:
+             GET, HEAD, CONNECT, OPTIONS, TRACE
+             -->
+            <xsd:enumeration value="POST" />
+            <xsd:enumeration value="PUT" />
+            <xsd:enumeration value="DELETE" />
+            <xsd:enumeration value="PATCH" />
+        </xsd:restriction>
+    </xsd:simpleType>
+
+    <xsd:element name="httpRequest">
+        <xsd:complexType>
+            <xsd:all>
+                <xsd:element name="method" type="httpMethods" />
+                <xsd:element name="path" type="xsd:string" />
+                <xsd:element name="body" type="xsd:string" />
+            </xsd:all>
+        </xsd:complexType>
+    </xsd:element>
+
+</xsd:schema>

src/test/java/liquibase/ext/opensearch/OpenSearchLiquibaseIT.java

@@ -33,11 +33,18 @@ public void itCreatesTheChangelogAndLockIndices() {
 
     @SneakyThrows
     @Test
-    public void itExecutesAHttpRequestAndCreatesTheIndex() {
+    public void itExecutesAHttpRequestAndCreatesTheIndexWithYAMLChangelog() {
         this.doLiquibaseUpdate("liquibase/ext/changelog.httprequest.yaml");
         assertThat(this.indexExists("testindex")).isTrue();
     }
 
+    @SneakyThrows
+    @Test
+    public void itExecutesAHttpRequestAndCreatesTheIndexWithXMLChangelog() {
+        this.doLiquibaseUpdate("liquibase/ext/changelog.httprequest.xml");
+        assertThat(this.indexExists("xmltestindex")).isTrue();
+    }
+
     @SneakyThrows
     @Test
     public void itHandlesReRuns() {

src/test/resources/liquibase/ext/changelog.httprequest.xml

@@ -0,0 +1,27 @@
+<databaseChangeLog
+        xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
+        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+        xmlns:opensearch="http://www.liquibase.org/xml/ns/opensearch"
+        xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-4.29.xsd
+        http://www.liquibase.org/xml/ns/opensearch http://www.liquibase.org/xml/ns/opensearch/liquibase-opensearch-1.0.xsd">
+
+    <changeSet id="10000" author="test" labels="XMLhttpRequestLabel" context="httpRequestContext">
+        <comment>httpRequestComment</comment>
+        <opensearch:httpRequest>
+            <opensearch:method>PUT</opensearch:method>
+            <opensearch:path>/xmltestindex</opensearch:path>
+            <opensearch:body>
+                {
+                    "mappings": {
+                        "properties": {
+                            "testfield": {
+                                "type": "text"
+                            }
+                        }
+                    }
+                }
+            </opensearch:body>
+        </opensearch:httpRequest>
+    </changeSet>
+
+</databaseChangeLog>