check that issued credentials work in token endpoint

[ltucker]

• adds an integration test to check that credentials issued from the /client endpoint can be used in the token endpoint to create an appropriate access token.

[github-actions]

Go test coverage

STATUS	ELAPSED	PACKAGE	COVER	PASS	FAIL	SKIP
🟢 PASS	1.19s	github.com/netboxlabs/diode/diode-server/auth	43.7%	31	0	0
🟢 PASS	1.02s	github.com/netboxlabs/diode/diode-server/authutil	81.1%	12	0	0
🟢 PASS	0.01s	github.com/netboxlabs/diode/diode-server/dbstore/postgres	0.0%	0	0	0
🟢 PASS	0.01s	github.com/netboxlabs/diode/diode-server/errors	0.0%	0	0	0
🟢 PASS	1.73s	github.com/netboxlabs/diode/diode-server/ingester	80.3%	9	0	0
🟢 PASS	1.06s	github.com/netboxlabs/diode/diode-server/migrator	70.4%	4	0	0
🟢 PASS	3.39s	github.com/netboxlabs/diode/diode-server/netboxdiodeplugin	83.9%	27	0	0
🟢 PASS	1.67s	github.com/netboxlabs/diode/diode-server/reconciler	70.7%	38	0	0
🟢 PASS	1.02s	github.com/netboxlabs/diode/diode-server/reconciler/applier	85.7%	1	0	0
🟢 PASS	0.01s	github.com/netboxlabs/diode/diode-server/reconciler/changeset	0.0%	0	0	0
🟢 PASS	1.35s	github.com/netboxlabs/diode/diode-server/reconciler/differ	55.8%	6	0	0
🟢 PASS	1.02s	github.com/netboxlabs/diode/diode-server/server	85.7%	14	0	0
🟢 PASS	0.00s	github.com/netboxlabs/diode/diode-server/telemetry	0.0%	0	0	0
🟢 PASS	1.01s	github.com/netboxlabs/diode/diode-server/version	100.0%	1	0	0

Total coverage: 52.8%