api: Support CORS

[mitjat]

Resolves #249

@lukaw3d if you're willing to test the frontend against this branch, the following should bring up a functional API server (backed by an empty DB):

make postgres
make oasis-indexer && ./oasis-indexer --config=config/local-dev.yml serve

Sample output:

$ curl -i -H 'Origin: example.com' 'localhost:8008/v1/consensus/blocks?limit=1'
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json
Vary: Origin
Date: Mon, 23 Jan 2023 12:48:57 GMT
Content-Length: 162

...

[lukaw3d]

Works!