Enhanced errors from API on validation failures

__Proposal:__
I would like to have errors that I can use to provide enhanced feedback in a programmatic way.

__Current behavior:__
Early exits (only ever 1 error). If there are multiple validation failures, then you will get first one encountered returned but none of the others.

__Desired behavior:__
I'll draw it out with an example, using a POST to `/api/v2/buckets` with a request body that fails to pass validation:

req body

```json
{
"name": "_reservedName",
"retentionRules": [
{
"type": "expire",
"everySeconds": -1,
}
]
}
```

This would fail both the name validation (can't have `_` prefix in bucket names, rserved for system buckets), and fails the retentionRules with the duration being less than 1 second.

I'd like to have a reply something akin too:

```json
{
"code": "unprocessable entity",
"errors": [
{
"field": "name",
"reason": "name must not be prefixed with \"_\""
},
{
"field": "retentionRules",
"reason": "expiration seconds must be greater than or equal to one second"
}
]
}
```

Something that I can use to inform the user and or my system in a programattic way.

__Alternatives considered:__
Adding `;` delimited errors as one big string. Doesn't help anyone who wants to do things programatticaly though

__Use case:__
Primarily to make it easy for users to consume the influxdb platform APIs and create simple interfaces on top of it.

Start by tracing validation for POST /api/v2/buckets and how its current response returns only the first failure. Compare the proposed structured response with the example failures for name and retentionRules; done means the API reports multiple validation errors in a programmatically consumable format.

Written by the indexing model from the issue text.