feat: namespace's connection announcement

[heiytor]

The connection announcement of a namespace is a custom UTF-8 valid
string provided by the user, with a maximum length of 127 characters,
which which can be written in a SSH session.

The SSH server must decide when to write this message in an established
connection and must skip this stage if the connection announcement is an
empty string.

A user can set the connection announcement by making a PUT request to
the existing /api/namespaces/:tenant route. The announcement must
leave under's settings.connection_announcement body:

A user can set the connection announcement by making a PUT request to
the existing /api/namespaces/:tenant route. The announcement must be
placed under the settings.connection_announcement field:

{
    "settings": {
        "connection_announcement": "My awesome connection announcement"
    }
}

NOTE: An empty string can be sent in this request to disable this
feature.

To retrieve the announcement, users can make a GET request to
api/namespaces/:tenant, where the announcement will be available in
settings.connection_announcement.

To retrieve the announcement, users can make a GET request to
api/namespaces/:tenant, where the announcement will be available in
settings.connection_announcement.

There are also two important things to notice:

1. A new method NamespaceLookup is introduced in the internal client,
   which can help the SSH session retrieve the connection announcement.
   The server must decide when to write this message in the session.

2. A new migration with ID 64 is added to automatically set the connection
   announcement to an empty string for already existing namespaces.

---

Progress:

• Edit the announcement
• Retrieve the announcement
• Write the announcement
• Store/Service tests
• Define the rules for the announcement (e.g., max length, allowed characters, etc.)
• Define the required user role to edit the announcement (the prototype suggests owner)
• Define the storage location for the announcement (the prototype suggests settings.connection_announcement)

[otavio]

Please fix CI issues.

[heiytor]

Please fix CI issues.

Fixed! It was an mock interface issue.