Test + document + refactor use of anyone in a mailbox ACL entry key

[chibenwa]

What

{using: [urn:ietf:params:jmap:mail, urn:ietf:params:jmap:core, urn:apache:james:params:jmap:mail:shares],
methodCalls: [[Mailbox/set, {accountId: 548575abf99bc986f5f2318a4ab2ca22790513380dd5beb32daf004934e1635d,
update: {58fb3d50-8f84-11ef-87be-c1d0831d0b43: {sharedWith: {anyone: [r]}}}}, c0]]}

CF linagora/tmail-flutter#3241

• Test showing this corresponds well to the anyone entry-key and not to the entry key of a user anyone.
• Document this into the urn:apache:james:params:jmap:mail:share documentation
• And finally a little refactoring in order to make this explicit in the type system of the JMAP datamodel (leverage sealed trait?)