Base64URL needs to be more clearly specified

The spec references the Base64URL format in a number of places. However, depending on the implementation there are some discrepancies in how encoding/decoding works.

According to [RFC4648](https://datatracker.ietf.org/doc/html/rfc4648) padding may or may not be used. In section [3.2](https://www.rfc-editor.org/rfc/rfc4648.html#section-3.2) there is a note on the unpadded implementation, sometimes referred to as a raw encoding.

The examples in the specification around long form DID use the version without padding. The spec language should be more specific to capture this nuance.