fix: init introspect http client once

[pike1212]

Related issue

#712

Proposed changes

Only initializes oauth client once during config stage and doesn't initialize when creating new introspector.

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
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• I have added tests that prove my fix is effective or that my feature
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• I have added or changed the documentation.

[pike1212]

It looks like the tests pass different configs to the same authenticator? Is that a valid situation when actually deploying though?

[aeneasr]

Awesome, thank you for your contribution!

[aeneasr]

Shit, I merged this too soon. You cant cache the client for all the configs. Reverting.

[aeneasr]

You need one cache per config or else all configs have the same client