[SB-1321] Enable/disable target at runtime

[m-cote]

New endpoints:

GET /admin/targets
returns a list of configured RPCs
Example response:

[
    {
        "name": "infura",
        "disabled": true,
        "blockNumber": 10534142
    },
    {
        "name": "QuickNode",
        "disabled": false,
        "blockNumber": 10534142
    }
]

POST /admin/targets/{{targetName}}
Disables or enables the RPC
Path parameters: targetName -- name of the target from the configuration
Request body:

{
    "disabled": true
}