Insert trigger function: If query affects no rows then return `null`
Insert trigger function is modified to have
`IF r IS NULL THEN RETURN null; ELSE RETURN r; END IF;` in return statement.
The API:
1. HGE has `--jwt-secret` flag or `HASURA_GRAPHQL_JWT_SECRET` env var. The value of which is a JSON.
2. The structure of this JSON is: `{"type": "<standard-JWT-algorithms>", "key": "<the-key>"}`
`type` : Standard JWT algos : `HS256`, `RS256`, `RS512` etc. (see jwt.io).
`key`:
i. Incase of symmetric key, the key as it is.
ii. Incase of asymmetric keys, only the public key, in a PEM encoded string or as a X509 certificate.
3. The claims in the JWT token must contain the following:
i. `x-hasura-default-role` field: default role of that user
ii. `x-hasura-allowed-roles` : A list of allowed roles for the user. The default role is overriden by `x-hasura-role` header.
4. The claims in the JWT token, can have other `x-hasura-*` fields where their values can only be strings.
5. The JWT tokens are sent as `Authorization: Bearer <token>` headers.
---
To test:
1. Generate a shared secret (for HMAC-SHA256) or RSA key pair.
2. Goto https://jwt.io/ , add the keys
3. Edit the claims to have `x-hasura-role` (mandatory) and other `x-hasura-*` fields. Add permissions related to the claims to test permissions.
4. Start HGE with `--jwt-secret` flag or `HASURA_GRAPHQL_JWT_SECRET` env var, which takes a JSON string: `{"type": "HS256", "key": "mylongsharedsecret"}` or `{"type":"RS256", "key": "<PEM-encoded-public-key>"}`
5. Copy the JWT token from jwt.io and use it in the `Authorization: Bearer <token>` header.
---
TODO: Support EC public keys. It is blocked on frasertweedale/hs-jose#61
