Add support for Q Business Apps integrated with IdC.

[harshav-aws]

Below changes enable the Gateway to work with Q Business Apps integrated with IdC.

Issue #33

Description of changes:

These changes enable the Gateway to work with Q Business Apps integrated with Identity Center (IdC).

• The gateway registers the Amazon Q Business Slack Gateway as an OpenID Connect (OIDC) app with Okta (or other OIDC compliant Identity Providers).
• This registration allows the gateway to invoke the Q Business ChatSync API on behalf of the end-user.
• The gateway provisions an OIDC callback handler for the Identity Provider (IdP) to return an authorization code after the end-user authenticates using the authorization grant flow.
• The callback handler exchanges the authorization code for IAM session credentials through a series of interactions with the IdP, IdC, and AWS Security Token Service (STS).
• The IAM session credentials, which are short-lived (15-minute duration), are encrypted and stored in a DynamoDB table along with the refresh token from the IdP.
• The IAM session credentials are then used to invoke the Q Business ChatSync and PutFeedback APIs.
• If the IAM session credentials expire, the refresh token from the IdP is used to obtain new IAM session credentials without requiring the end-user to sign in again.

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[harshav-aws]

Merging the changes after successful manual testing. Received verbal approval from @rstrahan.

[harshav-aws]

Merging the changes after successful manual testing. Received verbal approval from @rstrahan.