Remove requestDigest from Context schema in beckn.yaml

**Body:**

The `requestDigest` property in the `Context` schema (`api/v2.0.0/beckn.yaml`) should be removed along with the `RequestDigest` schema definition.

The callback binding mechanism is handled by the `request-signature` field in the Authorization header signing string (NFH-004 §6). The `requestDigest` object in Context is therefore redundant and should not be part of the final spec.

**Changes required:**
- Remove `requestDigest` property from the `Context` schema
- Remove the `RequestDigest` schema definition from `components/schemas`
- Remove the reference to `requestDigest` in the `Context` description text