Introduce PortManager to manage service ports

What does this Merge Request do and why?

This MR introduces GDK::PortManager which:

• Is the single source of truth for the TCP ports the GDK uses
• Validates there isn't a port clash when attempting to change a port's value
• Allows each TCP port to be inspected to determine if they're in use (not yet wired up)

Examples

$ gdk config set port 9000
❌️ ERROR: Value '9000' for setting 'port' is not a valid port - Port 9000 is already allocated for service 'object_store'.

# edited gdk.yml to set `port: 9000`

$ gdk config get port
❌️ ERROR: Your gdk.yml is invalid.

Value '9000' for setting 'port' is not a valid port - Port 9000 is already allocated for service 'object_store'.

$ gdk config get port
❌️ ERROR: Your gdk.yml is invalid.

Service 'port_not_defined' is unknown, please add to GDK::PortManager::DEFAULT_PORTS_FOR_SERVICES

Closes: #1581 (closed)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.