feat: allow for configuring `dissolveNamespaces` in client

[ThorstenSuckow]

With conjoon 2.0, the general behavior of reading out mailboxes is changed in such a way that "subscriptions" for mailboxes are specified in a filter-string (to keep the backend protocol agnostic and allow for "subscriptions" event if the messaging solution does not support them).

This has an impact on the way the TreeFolderBuilder assembles tree-stuctures - previously those "namespaces" where considered as the root-folders from which the MailFolderList was assembled - now they are considered as namespaces to dissolve.

The value for this option is now hardcoded to ["INBOX", "[GMAIL]"] as default, but should be made available with an option value for a query string that gets passed to the MailFolder-api.

[ThorstenSuckow]

The default values for dissolveNamespaces in Conjoon\MailClient\Data\MailAccount should be removed. Settings must be made available using the imapserver-config or the account data sent by the client.

[ThorstenSuckow]

The default values for dissolveNamespaces in Conjoon\MailClient\Data\MailAccount should be removed. Settings must be made available using the imapserver-config or the account data sent by the client.

done with cb3c0ea