feat: add configurable OIDC provider display name

[lusu007]

What changed

This adds an OIDC_PROVIDER_NAME setting that controls the user-facing OIDC provider name shown in Sentry.
By default the provider is still displayed as OIDC. Installations can now set, for example:

OIDC_PROVIDER_NAME = "Acme Corp SSO"

The provider key remains stable as oidc, so existing Sentry template classes such as .provider-logo.oidc and .btn-login-oidc remain predictable for custom styling.

Why this PR

This supersedes #74. That PR was opened by Copilot and still contained generated-agent footer text and documentation that implied the icon CSS class was derived from the display name. I opened this replacement PR from a reviewed branch so the implementation and docs match Sentry's actual template behavior: the display text is configurable, while the CSS hook remains the provider key.

Details

• Uses the same display name in the OIDC configure view.
• Documents the stable .provider-logo.oidc icon hook.
• Adds an example icon and regression coverage for the configure view context.

Fixes #19.

Validation

• uv run ruff check .
• uv run ruff format --check .
• Previously verified uv run pytest tests/test_provider.py locally with the required services.
• Also smoke-tested a temporary local Sentry instance

[CLAassistant]

All committers have signed the CLA.

[gemini-code-assist]

Code Review

This pull request introduces the OIDC_PROVIDER_NAME configuration option to allow customization of the OIDC provider's display name in the Sentry UI, along with documentation and an example SVG icon for custom branding. The review feedback suggests falling back to ISSUER in oidc/constants.py to prevent a regression for existing installations that relied on OIDC_ISSUER as the display name. Additionally, the documentation in examples/README.md should be updated to provide the correct base64 command for macOS users, as the GNU-specific -w flag is not supported on BSD-based systems.

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[max-wittig]

Sorry for the late response @lusu007. Would you mind rebasing again?

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.