Make ConversionService customisable

- Add ShellConversionServiceSupplier interface to define
  shell spesific ConversionService so that we don't
  get trouble with other services in boot.
- Fixes #435

spring-shell-core/src/main/java/org/springframework/shell/config/ShellConversionServiceSupplier.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.shell.config;
+
+import java.util.function.Supplier;
+
+import org.springframework.core.convert.ConversionService;
+
+/**
+ * Interface to provide shell spesific {@link ConversionService}.
+ *
+ * @author Janne Valkealahti
+ */
+public interface ShellConversionServiceSupplier extends Supplier<ConversionService> {
+}