Include external id in session cache key

[mtanda]

If external id is specified, API call from datasource with invalid external id, should fail.
https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_create_for-user_externalid.html

Currently, Grafana doesn't include external id in cache key.
It may cause issue.

If Grafana has two datasource settings.

• datasource A with valid external ID
• datasource B with invalid external ID

Grafana will create one session cache from these two datasource setting.
If Grafana cache datasource A, API call which using datasource B will success (it should fail).

I suggest to include external id in cache key.

[andresmgot]

Thanks for the PR @mtanda, we are aware of this issue. It happens with more fields, not only the ExternalID (also the SecretKey, for example). We have discussed this in grafana/athena-datasource#144 (comment) but we didn't get to it yet. Our idea is to use a hash of all the settings so we catch any change.

[mtanda]

@andresmgot Thanks for reply :-)
The discussed issue seems to be similar issue. By using hash of all the settings will solve my issue too.
So I close my PR.