Fix remote spec handling and hash calculation

Created by: johngray

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

Current implementataion of maven plugin has the feature of skipping generation if no changes was detected in schema file. While the skipping itself is applied only when the skipIfSpecIsUnchanged option is present in the configuration file - hash calculation is calculated every time:

File storedInputSpecHashFile = getHashFile(inputSpecFile);
ByteSource inputSpecByteSource =
    inputSpecFile.exists()
        ? Files.asByteSource(inputSpecFile)
        : CharSource.wrap(ClasspathHelper.loadFileFromClasspath(inputSpecFile.toString().replaceAll("\\\\","/")))
                        .asByteSource(Charsets.UTF_8);
String  inputSpecHash =inputSpecByteSource.hash(Hashing.sha256()).toString();

This makes impossible to use remote spec files in pom configuration. FileNotFoundException is thrown at the end of generation phase despite that all classes generated fine. Generator itself are working great with remote specs.

My proposition in this PR is to fix this issue and allow the generator to work with remote spec files:

URL inputSpecRemoteUrl = inputSpecRemoteUrl();
File inputSpecTempFile = inputSpecFile;
if (inputSpecRemoteUrl != null) {
    inputSpecTempFile = File.createTempFile("openapi-spec", ".tmp");
    ReadableByteChannel readableByteChannel = Channels.newChannel(inputSpecRemoteUrl.openStream());
    
    FileOutputStream fileOutputStream = new FileOutputStream(inputSpecTempFile);
    FileChannel fileChannel = fileOutputStream.getChannel();

    fileChannel.transferFrom(readableByteChannel, 0, Long.MAX_VALUE);
}

ByteSource inputSpecByteSource =
    inputSpecTempFile.exists()
        ? Files.asByteSource(inputSpecTempFile)
        : CharSource.wrap(ClasspathHelper.loadFileFromClasspath(inputSpecTempFile.toString().replaceAll("\\\\","/"))).asByteSource(Charsets.UTF_8);

return inputSpecByteSource.hash(Hashing.sha256()).toString();

To fix #1792 (closed)