feat: implement support for OAuth2 authentication in the OTLP output

[TeodorSAP]

Description

Changes proposed in this pull request (what was done and why):

• Use OTel's oauth2client extension to support OAuth2 in the OTLP output
• Adapt configuration builders and secret management accordingly
• Implement UTs for the new configuration flow

⚠️ DISCLAIMER: GRPC output without proper TLS configuration is not supported by the OTel collector when OAuth2 is enabled.

Left TODO:

• E2E tests
• Documentation
• Validation for GRPC without proper TLS configuration scenario

Changes refer to particular issues, PRs or documents:

• Support for oauth2 based authentication in the OTLP output #2657
• Requires extension enablement in collector image: feat: add oauth2client OTel extensions to builder config opentelemetry-collector-components#433

Traceability

• The PR is linked to a GitHub issue.
• The follow-up issues (if any) are linked in the Related Issues section.
• If the change is user-facing, the documentation has been adjusted.
• If a CRD is changed, the corresponding Busola ConfigMap has been adjusted.
• The feature is unit-tested.
• The feature is e2e-tested.

[TeodorSAP]

The following Busola integration has been implemented:

Details View / Authentication Panel

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Form View / Basic Authentication

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Form View / OAuth2 Authentication

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Form View / OAuth2 Authentication (expanded)