Support Base64Url format

[tbombach]

AutoRest modeler supports a base64url format for string types. This is implemented in some languages, but requires serialization/deserialization code in some remaining languages.

This issue: #805 tracked the changes needed in the modeler. It was implemented in C# in this PR: #878

Here is the specification for Base64Url format: https://tools.ietf.org/html/rfc4648#page-7

[tbombach]

@jianghaolu I've assigned this to you for the Java implementation, but please re-assign to someone else if they're going to end up doing this work.

[jianghaolu]

Thanks @tbombach

[pomortaz]

@jianghaolu there are issues with the solution when generating the code.

Error with :

    public byte[] result() {
        if (this.result == null) {
            return null;
        }
        return this.result.getbyte[]();
    } 

Error for not accepting byte[] in the constructor of Base64Url:

    public KeyOperationsParameters withValue(byte[] value) {
        this.value = new Base64Url(value);
        return this;
    } 

[jhendrixMSFT]

This is tracked in Azure/azure-sdk-for-go#2111.