Deterministic Base64 support

This issue is being created to supersede similarly reported issues and represent a canonical issue to enable the following:

1. Check to see if java.util.Base64 is available at runtime using Classes.isAvailable (or similar), and if so, use that. This is the best/fastest one available to most default environments and should be used whenever possible.

   java.util.Base64 is available on JDK >= 8 and Android level >= 26

2. If java.util.Base64 is not available:

   1. If the runtime enviornment is Android, use android.util.Base64. This is available on Android API level >= 8 (i.e. >= Android 2.2, “Froyo”)

   2. Otherwise, assume < JDK8, and use javax.xml.bind.DatatypeConverter. JDK 7 has this included automatically.

3. In any case, enable JwtBuilder and JwtParser methods that allow a user to specify their own Base64 TextCodec if/when desired, e.g. Jwts.parser().setBase64Codec(myBase64Codec) This last option would allow the user to enable Base64 support in any environment that doesn’t match 1 and 2 above.

4. Ensure any codec implementation will throw exceptions on invalid inputs when possible. See #143 as an example. Exceptions should be thrown when possible to indicate invalid input, wrapping/propagating the original codec exception.

Implementation note: Perhaps instead of using javax.xml.bind.DatatypeConverter - which doesn’t always fail on invalid inputs (see #143), we should embed the Commons Codec or MigBase64 implementation.

[lhazlewood]

This has been released in 0.10.0.

[lhazlewood]

@azagniotov I’ve used migbase64 in production environments before, so I’m not too worried about that. That said, I’m adding tests for it anyway to ensure we still keep 100% coverage.