Update spec to include changes to the echcheck test (#297)

## Checklist

- [x] I have read the [contribution
guidelines](https://github.com/ooni/spec/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request: <!-- add URL here -->
- [x] related ooni/probe-cli pull request:
ooni/probe-cli#1658
- [x] If I changed a spec, I also bumped its version number and/or date

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## Description

Please, insert here a more detailed description.

data-formats/df-006-tlshandshake.md

@@ -32,6 +32,7 @@ code. See this directory's [README](README.md) for the basic concepts.
     "no_tls_verify": false,
     "peer_certificates": [],
     "server_name": "example.com",
+    "echconfig": "",
     "t0": 1.001,
     "t": 1.11,
     "tags": [],
@@ -73,6 +74,18 @@ to verify the server's X.509 certificate. Note that, when this field contains an
 address rather than a domain name, the corresponding value is not included in the TLS
 ClientHello as described by [RFC 6066, Section 3](https://datatracker.ietf.org/doc/html/rfc6066#section-3);
 
+- `outer_server_name`: (`string`; optional): server name used in the
+`ClientHelloOuter` when [TLS ECH](https://www.ietf.org/archive/id/draft-ietf-tls-esni-22.html)
+is being used. When this is set, the `server_name` field indicates the field
+used inside of the encrypted client hello.
+
+- `echconfig`: (`string`; optional): ECHConfig as defined in [TLS ECH
+  Spec](https://www.ietf.org/archive/id/draft-ietf-tls-esni-22.html#name-encrypted-clienthello-confi)
+base64 encoded as it would be presented inside of an SVCB HTTPS SvcParam as per
+[RFC9460](https://www.rfc-editor.org/rfc/rfc9460.html). In the event that only
+[GREASE ECH](https://www.ietf.org/archive/id/draft-ietf-tls-esni-22.html#name-grease-psk) is being used, it will contain the
+string literal `GREASE`.
+
 - `t0` (`float`): number of seconds elapsed since `measurement_start_time`
 measured in the moment in which we started the operation (`t - t0` gives you
 the amount of time spent performing the operation);