feat: lazy load jsonParser and yamlParser FlagFileParser (#35)

I want to be able to use the thin jar without the snakeyaml dependency,
since I don't need to parse yaml files, just json files.

If I try that I get the following exception:

```
Cause: java.lang.ClassNotFoundException: org.yaml.snakeyaml.error.YAMLException
at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:476)
at sbt.internal.ManagedClassLoader.findClass(ManagedClassLoader.java:103)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:594)
at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
at com.launchdarkly.sdk.server.integrations.FileDataSourceParsing$FlagFileParser.<clinit>(FileDataSourceParsing.java:76)
at com.launchdarkly.sdk.server.integrations.FileDataSourceImpl$DataLoader.load(FileDataSourceImpl.java:241)
at com.launchdarkly.sdk.server.integrations.FileDataSourceImpl.reload(FileDataSourceImpl.java:108)
at com.launchdarkly.sdk.server.integrations.FileDataSourceImpl.start(FileDataSourceImpl.java:92)
at com.launchdarkly.sdk.server.LDClient.<init>(LDClient.java:239)
```

This PR avoids loading the `YamlFlagFileParser` by using a holder class
idiom for lazy initialization of a static field.

**Requirements**

- [ ] I have added test coverage for new or changed functionality
- Existing tests still pass, but I don't see a way to add unit tests for
this functionality. I published this library locally and tested it on my
project successfully.
- [x] I have followed the repository's [pull request submission
guidelines](../blob/main/CONTRIBUTING.md#submitting-pull-requests)
- [ ] I have validated my changes against all supported platform
versions

**Related issues**

n/a

**Describe the solution you've provided**

The solution is to lazy load the `YamlFlagFileParser` (and the
`JsonFlagFileParser`) so they are only initialised if needed

**Describe alternatives you've considered**

n/a

**Additional context**

n/a

Author: jtjeferreira

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/integrations/FileDataSourceParsing.java

@@ -72,14 +72,24 @@ static final class FlagFileRep {
   }
 
   static abstract class FlagFileParser {
-    private static final FlagFileParser jsonParser = new JsonFlagFileParser();
-    private static final FlagFileParser yamlParser = new YamlFlagFileParser();
+    static class JsonParserHolder {
+      private static final JsonFlagFileParser INSTANCE = new JsonFlagFileParser();
+      static JsonFlagFileParser getInstance() {
+        return INSTANCE;
+      }
+    }
+    static class YamlParserHolder {
+      private static final YamlFlagFileParser INSTANCE = new YamlFlagFileParser();
+      static YamlFlagFileParser getInstance() {
+        return INSTANCE;
+      }
+    }
 
     public abstract FlagFileRep parse(InputStream input) throws FileDataException, IOException;
 
     public static FlagFileParser selectForContent(byte[] data) {
       Reader r = new InputStreamReader(new ByteArrayInputStream(data));
-      return detectJson(r) ? jsonParser : yamlParser;
+      return detectJson(r) ? JsonParserHolder.getInstance() : YamlParserHolder.getInstance();
     }
 
     private static boolean detectJson(Reader r) {