[vtadmin] full dynamic cluster config

[ajm188]

Description

This PR changes the unmarshalling technique used for dynamic cluster configs, to support a fully-configured dynamic cluster with parity to what you can do on the CLI.

The most notable changes for end users are:

• you must specify an id key rather than a name key. you should also specify a name key, but it's not strictly required.
• you must specify your discovery implementation name. this means you can use dynamic clusters with other discovery implementations (like consul!) "for free"
• when using dynamic discovery with a dynamic cluster (boy that's a lotta "dynamic"), you are technically going through a double layer (mmmm, burritosssss) of json unmarshalling, which means your discovery-dynamic-discovery option must be properly quoted and escaped (see the tests).

Below are some screenshots where I uploaded first the original dynamic cluster, and then a dynamic cluster with some configuration options. Here are the configs used:

minimal dynamic cluster

{
    "id": "dynamic",
    "name": "my-dynamic-cluster",
    "discovery": "dynamic",
    "discovery-dynamic-discovery": "{\"vtctlds\": [ { \"host\": { \"fqdn\": \"localhost:15000\", \"hostname\": \"localhost:15999\" } } ], \"vtgates\": [ { \"host\": {\"hostname\": \"localhost:15991\" } } ] }"
}

configured

{
    "id": "dynamic2",
    "name": "my-configured-dynamic-cluster",
    "discovery": "dynamic",
    "discovery-dynamic-discovery": "{\"vtctlds\": [ { \"host\": { \"fqdn\": \"localhost:15000\", \"hostname\": \"localhost:15999\" } } ], \"vtgates\": [ { \"host\": {\"hostname\": \"localhost:15991\" } } ] }",
    "vtsql-discovery-timeout": "1h"
}

Pulling up the debug page for the configured cluster, we can see that the vtsql discovery timeout did indeed get set to 1h, rather than the default 100ms:

 curl -s localhost:14200/debug/cluster/dynamic2 | jq '.' | grep timeout      
      "discovery-timeout": "1h"
      "discovery_timeout": 100000000,
      "discovery_timeout": 3600000000000,

Related Issue(s)

Checklist

• Should this PR be backported? no
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[notfelineit]

🎉