feat: replace api key with oauth2

[leoparente]

This pull request introduces significant changes to the Diode SDK by replacing API key-based authentication with OAuth2 client credentials. The most important changes include updating environment variables, modifying the client structure and methods, and updating tests to reflect these changes.

Authentication Changes:

• README.md: Updated environment variables to include DIODE_CLIENT_ID and DIODE_CLIENT_SECRET for OAuth2 authentication.
• diode/client.go: Removed DIODE_API_KEY and added DIODE_CLIENT_ID and DIODE_CLIENT_SECRET environment variables.
• diode/client.go: Replaced getAPIKey function with getClientID and getClientSecret functions to retrieve OAuth2 credentials.
• diode/client.go: Added diodeAuthentication struct and methods to handle OAuth2 token fetching and updating client metadata.

Client Structure Changes:

• diode/client.go: Updated GRPCClient struct to include clientID and clientSecret fields instead of apiKey.
• diode/client.go: Modified NewClient function to initialize clientID and clientSecret instead of apiKey.

Test Updates:

• diode/client_test.go: Updated tests to reflect the changes from API key to OAuth2 client credentials. This includes modifying test cases to use clientID and clientSecret. [1] [2] [3] [4] [5] [6] [7]

Example Update:

• examples/main.go: Updated example usage to include WithClientID and WithClientSecret instead of WithAPIKey.

[github-actions]

Go test coverage

STATUS	ELAPSED	PACKAGE	COVER	PASS	FAIL	SKIP
🟢 PASS	1.21s	github.com/netboxlabs/diode-sdk-go/diode	20.0%	212	0	0

Total coverage: 92.6%

[mfiedorowicz]

LGTM 🙇 , one cov related question