Adding server_uri for dispatcher node

[aaronchongth]

New feature implementation

Implemented feature

Related to open-rmf/rmf_ros2#355.
Gives the dispatcher node websocket access to servers

[arjo129]

LGTM

[mxgrey]

By changing the default value for server_uri, this PR has had the unpleasant side effect of causing these error messages to be blasted out once per second if a user is running a simulation without using the api-server:

[fleet_adapter-16] [ERROR] [1717560956.734331412] [tinyRobot_fleet_adapter]: Connection not yet established
[fleet_adapter-16] [INFO] [1717560956.757289553] [tinyRobot_fleet_adapter]: Attempting to connect to ws://localhost:8000/_internal
[fleet_adapter-16] [ERROR] [1717560956.758046354] [tinyRobot_fleet_adapter]: Connection to ws://localhost:8000/_internal failed. Reason:
[fleet_adapter-16]  > URI: ws://localhost:8000/_internal
[fleet_adapter-16] > Status: Closed
[fleet_adapter-16] > Remote Server: None Specified
[fleet_adapter-16] > Error/close reason: Underlying Transport Error
[fleet_adapter-16] 
[fleet_adapter-16] [ERROR] [1717560956.758128734] [tinyRobot_fleet_adapter]: Connection lost
[fleet_adapter-16] > Reconnecting in 1s
[fleet_adapter-16] > Host: ws://localhost:8000/_internal
[rmf_task_dispatcher-13] [INFO] [1717560956.953390910] [rmf_dispatcher_node]: Attempting to connect to ws://localhost:8000/_internal
[rmf_task_dispatcher-13] [ERROR] [1717560956.954344271] [rmf_dispatcher_node]: Connection to ws://localhost:8000/_internal failed. Reason:
[rmf_task_dispatcher-13]  > URI: ws://localhost:8000/_internal
[rmf_task_dispatcher-13] > Status: Closed
[rmf_task_dispatcher-13] > Remote Server: None Specified
[rmf_task_dispatcher-13] > Error/close reason: Underlying Transport Error
[rmf_task_dispatcher-13] 
[rmf_task_dispatcher-13] [ERROR] [1717560956.954460261] [rmf_dispatcher_node]: Connection lost
[rmf_task_dispatcher-13] > Reconnecting in 1s
[rmf_task_dispatcher-13] > Host: ws://localhost:8000/_internal

I think we should be consistent and keep the default parameter value for server_uri as an empty string. Since this PR was merged, the only way for a user to indicate that they don't need a web connection is to launch like this:

$ ros2 launch rmf_demos_gz office.launch.xml server_uri:=\"\"

The user needs to figure out that

1. server_uri must be explicitly set to blank to stop the error messages
2. Escaped-quotes need to be used in order for the terminal command to express an empty string

I expect this to cause significant confusion for users.