logging action webhook calls

When `HASURA_GRAPHQL_LOG_LEVEL` is set to `debug`, then log the **request** to and **response** from the action webhook call. The log-type of this should be `action` (similar to `event-trigger` log-type for event trigger responses). So it will look like (similar to event trigger logs)

```
{
"type":"action",
"timestamp":"2020-03-18T06:20:35.373+0000",
"level":"debug",
"detail":{
"context":{"action_id":"5d8a8bed-024c-40ca-9521-287c330f83c2"}, # any kind of context (for e.g. action_id for async actions)
"request":{"url": "", "body":"{}", "headers": []},
"response":{"status":200,"body":"{}", "headers": []}
}
}
```

**Note on logging headers**:

- Request headers should be redacted based on whether the value is environment variable and should be over-rideable using `LOG_HEADERS_FROM_ENV` env flag. This infra already exists for Event Triggers -- see https://github.com/hasura/graphql-engine/blob/4d2dbe68fa244ea100356ca1f104f7fb1c6ee2ae/server/src-lib/Hasura/App.hs#L257 and https://github.com/hasura/graphql-engine/blob/4d2dbe68fa244ea100356ca1f104f7fb1c6ee2ae/server/src-lib/Hasura/Events/Lib.hs#L370

- Header `Authorization` should be redacted.

- Redacting response headers will be outside the scope of this spec. We can also choose to not log response headers till we spec this.

Start by comparing Hasura/App.hs and Hasura/Events/Lib.hs for the existing event-trigger logging and header-redaction behavior, then locate the action webhook request and response path. Done means debug-level logs use type action, include the request, response, and available context, redact Authorization and environment-derived request headers with LOG_HEADERS_FROM_ENV, and omit response-header redaction as specified.

Written by the indexing model from the issue text.