Add configurable provider name for custom icon support

[Copilot]

Users cannot customize the OIDC provider icon in Sentry's authentication selection UI because the provider name was dynamically set to the ISSUER value, making the CSS class unpredictable.

Changes

Configuration

• Added OIDC_PROVIDER_NAME setting (defaults to "OIDC")
• Decoupled provider display name from ISSUER (now used only for token validation)

Provider

• Changed OIDCProvider.name from ISSUER to PROVIDER_NAME constant
• Generates predictable CSS class: OIDC_PROVIDER_NAME = "My Company SSO" → .provider-logo.my-company-sso

Documentation & Example

• Added icon usage guide with three implementation options
• Included example SVG icon with user + lock symbol
• Documented CSS class generation rules

Usage

Configure the provider name in sentry.conf.py:

OIDC_PROVIDER_NAME = "Acme Corp SSO"

Add custom CSS to Sentry:

.provider-logo.acme-corp-sso {
    background-image: url('/static/acme-icon.svg');
}

Fixes #19

Original prompt

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This section details on the original issue you should resolve

<issue_title>custom icon </issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes custom icon  #19

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