Include RequestTimeout in marshal/unmarshal of ServiceResolverConfigE…

[cthain]

Description

This PR fixes a bug where we’re missing the custom JSON marshal and unmarshal for the ServiceResolverConfigEntry.RequestTimeout field. This has the effect that the config entry can be written successfully to Consul but it cannot be read back through the API.

Testing & Reproduction steps

Before the changes proposed in this PR:

$ consul agent -dev &> /dev/null &
$ consul config write -
Kind      = "service-resolver"
Name      = "frontend-se"
ConnectTimeout = "10s"
RequestTimeout = "15s"
Subsets = {
 v1 = {
   OnlyPassing = true
 }
 v2  = {
   OnlyPassing = true
 }
}^D
Config entry written: service-resolver/frontend-se
$ consul config read -kind service-resolver -name frontend-se
Error reading config entry service-resolver/frontend-se: json: cannot unmarshal string into Go struct field .RequestTimeout of type time.Duration

After these changes:

# setup as above
consul config read -kind service-resolver -name frontend-se
{
    "ConnectTimeout": "10s",
    "RequestTimeout": "15s",
    "Kind": "service-resolver",
    "Name": "frontend-se",
    "Partition": "default",
    "Namespace": "default",
    "Subsets": {
        "v1": {
            "OnlyPassing": true
        },
        "v2": {
            "OnlyPassing": true
        }
    },
    "CreateIndex": 22,
    "ModifyIndex": 22
}

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern