Add Base16 (Hex) and Base64 encoding and decoding capabilities

[Glusk]

At the moment, we're wired to an external dependency jakarta.xml.bind:jakarta.xml.bind-api just so that we can use DatatypeConverter. This dependency could be removed.

Java 8 introduced Base64 class and the hex encoding/decoding looks easy enough to implement.

The library currently only parses Base64 and Base16 strings. We should add default methods to Bytes that would encode byte sequences to Base64 and Base16 strings.

[Glusk]

Base64 encoding/decoding should be Basic.

If the user wants to read a Base64 MIME encoded file, he or she can do it this way in Java 9 or later:

byte[] decodedBytes = Base64.getMimeDecoder().wrap(/*InputStream*/).readAllBytes();

decodedBytes can then be further used by Caesar.

The procedure is analogous for encoding.

[Glusk]

Hex string need not be even length. As a consequence, different length strings may result in the same byte array.

This library should be able to parse all valid hex strings.

The simplest way to encode a byte array to an even length string:

// byte[] array = ...
StringBuilder sb = new StringBuilder();
for (byte b : array) {
    sb.append(String.format("%02X", b));
}
String result = sb.toString();

What to do with empty strings ("")?

[Glusk]

Base64 support will be dropped. It is easy enough for the user of this library to convert a Base64 string to a byte array (and vice versa) using the Java SE API version 8 or later.