Deterministic Base64 support

[lhazlewood]

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 Consider using a different Base64 decoder #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]

cc @RyanBard

[lhazlewood]

@RyanBard I don't think we'll need the JAXB dependency in the pom now based on the above fallback logic? That is, javax.xml.bind.DatatypeConverter would only be used in a JDK 7 environment which is guaranteed to exist in that JDK. Do you agree? Or might I be missing something?

[RyanBard]

@lhazlewood I agree.

[lhazlewood]

@RyanBard I'm just about to start work on this - I just wanted to make sure you haven't already so we're not stepping on each other's toes. Let me know.