Add function to clean a string to make it into a valid id

[ftomassetti]

LionWeb has strong limits on the characters that can be used in an id or a key. This means that someone generating IDs from various sources can end up producing illegal IDs. Just removing all the forbidden characters can lead to conflicts. So this problem could be solved in this library, to avoid different users having to solve it individually.

[enikao]

We should also add these two methods to the library, that encode/decode an arbitrary string to a valid LW id:

new String(Base64.getUrlDecoder().decode(lionWebId), StandardCharsets.UTF_8);
Base64.getUrlEncoder().withoutPadding().encodeToString(mpsId.toString().getBytes(StandardCharsets.UTF_8));

[ftomassetti]

Yes, they would be very useful!