OCM/OCM-MLS: Standardize on base64url

[mickenordin]

We use base64 in several places, in particular the invitation strings needs to be URL safe. This commit standardizes on base64url encoding in all places where we use base64, including in diagrams and in the IETF-OCM-MLS draft.

The example invite string:

YTU1YTk2NmUtMTVjMS00Y2I5LWEzOWQtNGU0YzU0Mzk5YmFmQGNsb3VkLmV4YW1wbGUub3Jn

can be decoded using standard GNU coreutils like this:

echo -n 'YTU1YTk2NmUtMTVjMS00Y2I5LWEzOWQtNGU0YzU0Mzk5YmFmQGNsb3VkLmV4YW1wbGUub3Jn' | basenc -d --base64url

and yields: a55a966e-15c1-4cb9-a39d-4e4c54399baf@cloud.example.org

[glpatcern]

Perfect, this is very unambiguous

[thibmeu]

much clearer. i don't know about the state of implementations. it might be worth checking if this is breaking and update them accordingly

[MahdiBaghbani]

I like it!

[MahdiBaghbani]

I have to remind us that this can become a bit misused as we have seen in the Reva based platforms other than the CERNBox in the past year's EOSC efforts.

while its fine for the invite to be base64, its definitely not fine for the actual OCM identifier of opaque_id@fqdn to be in base64 format. and even worse in <base64>@fqdn format.