Add a way to configure the preference of servers

Add a mechanism which allows the order of the servers to be selected based on a user-configurable parameter. For example, with servers A, B and C used in a service, the mechanism should allow:

[My-Service]

type=service

router=readconnroute

servers=A,B,C

preference=C,B,A

For readconnroute, this should select server C if it's valid, then B and finally A if no other server is available. Servers not included in the preference parameter would be considered after the ones defined in it.

A syntax could be added that could be used to group the servers. This allow finer control over how the servers are used:

[My-Service]

type=service

router=readconnroute

servers=A,B,C,D,E,F,G

preference=(C,D),A,(B,G),F

The servers would get selected in this order:

1. C or D
2. A
3. B or G
4. F

Original title: Add formal configuration option for allowing duplicate servers in MaxScale

Original description:

Currently, the MaxScale server allows the use of duplicate servers only when the debug=allow-duplicate-servers option is set in the configuration. This functionality should be enhanced to enable allowing duplicate servers through a standard, non-debug configuration option.

Steps to Reproduce:

1. Set up a MaxScale configuration with multiple servers that may have overlapping host/port combinations.
2. Attempt to configure MaxScale to allow duplicate servers using the debug option debug=allow-duplicate-servers.
3. Note that this feature is only accessible through a debug configuration setting.

Expected Behavior:

The system should allow the configuration of duplicate servers through a standard configuration option, such as allow_duplicate_servers, rather than relying on a debug option. This should be clearly documented and easily accessible for production use.

Proposed Solution:

1. Introduce a new configuration option, allow_duplicate_servers, in the MaxScale configuration file.
2. Update the server creation logic to check this new option instead of the debug flag.
3. Ensure that the new option is properly documented in the MaxScale configuration guide.
   Additional Information:

In version 2.3.12, it was possible to add the same server multiple times without needing a debug option. This feature should be reintroduced in a more formal and user-friendly manner.

Rationale:

Allowing duplicate servers via a standard configuration option improves usability and flexibility, making it easier for users to manage their server configurations without needing to enable debug settings. This change aligns with best practices for production environments and enhances the overall robustness of the MaxScale server management