Global Auth token variable causes Trino datasources to share authentication state

[Flgado]

The OAuth access token is stored as a global package-level variable, which means all datasource instances share the same token. This causes authentication issues when multiple Trino datasources are configured with different OAuth client_id / client_secret pairs or different authentication methods.

As a result:

1. User A configures a Trino datasource with `client1 / secret1 → obtains tokenA
2. User B configures another Trino datasource with client2 / secret2 → unexpectedly reuses tokenA instead of generating its own
3. When any token expires, datasource A may begin using datasource B’s token, or vice-versa (after refresh the token)

The root cause is this global variable:
https://github.com/trinodb/grafana-trino/blob/7b190bc0f359ceb3d432790a6dfed110aa324ce8/pkg/trino/client/client.go#L15C1-L18C2