Out of date documentation

### Subject of the issue
[DEVELOPER_GUIDE.MD](https://github.com/adobe/experience-platform-streaming-connect/blob/master/DEVELOPER_GUIDE.md) has out of date information:

https://github.com/adobe/experience-platform-streaming-connect/blob/a0502759683c81b0b4d03a0c00548242d558d314/DEVELOPER_GUIDE.md?plain=1#L40

aep.connection.auth.token.type should always be ```access_token``` however this is not the case if using OAUTH S2S. It should be ```oauth2_access_token```

https://github.com/adobe/experience-platform-streaming-connect/blob/a0502759683c81b0b4d03a0c00548242d558d314/DEVELOPER_GUIDE.md?plain=1#L42

aep.connection.auth.client.code is very non descript and unclear what this actually refers to. It would be appreciated if the definition of this variable could be clarified and made clearer. Ideally conforming to the same naming conventions as in Developer Console.

Open DEVELOPER_GUIDE.md and review the references around lines 40 and 42. Update the documented token type for OAUTH S2S and clarify what aep.connection.auth.client.code refers to, using the Developer Console naming convention as the guide. Done means both settings are accurately and clearly described.

Written by the indexing model from the issue text.