[oidc-middleware] Add oidcClientOptions

[denysoblohin-okta]

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• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Adding Tests
• Build related changes
• CI related changes
• Documentation changes
• Other... Please describe:

What is the current behavior?

Customer reports then default authorization for /token endpoint (client_secret_basic) doesn't work for some reason (related to configuration of his network probably) but client_secret_post works.

Issue Number: OKTA-379204

What is the new behavior?

Developer can pass custom options to OpenId Client constructor

Should resolve issue okta/samples-nodejs-express-4#105
by adding

    oidcClientOptions: {
      token_endpoint_auth_method: 'client_secret_post',
    }

in options for ExpressOIDC here: https://github.com/okta/samples-nodejs-express-4/blob/master/common/sample-web-server.js#L31

Does this PR introduce a breaking change?

• Yes
• No

Other information

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