SANTUARIO-525 Made Base64 line length and line separator configurable…

[peterdemaeyer]

• Made Base64 line length and separator configurable on XMLSecurityProperties.
• Used a thread-local on XMLUtils to hold the Base64 parameters per thread. The alternative is to pass them as parameters to XMLUtils.encode, which is what I started doing at first, but it lead to a lot more code changes since the parameters have to be passed everywhere through the whole call stack. The approach with the thread-local seemed like a good alternative, certainly no worse than the configuration with the system property.
• Removed some ad-hoc usages of the static calls to XMLUtils.ignoreLineBreaks and isIgnoreLineBreaks.
• Deprecated both static methods ignoreLineBreaks and isIgnoreLineBreaks, because the new mechanism for configuring Base64 parameters is far more flexible. I considered removing those methods, but I decided not to because they're part of the public API of Santuario and it's probably not a good idea to remove them just yet.
• Kept the default CRLF and system property for backward compatibility. I considered removing the system property, but decided not to because I didn't want to affect existing behavior too much. It would be best to remove it at some point though.
• Added unit tests, including a test for concurrent usage.