Add Gcore Image Stack image provider support

[stefashkaa]

Feature Title

Add Gcore Image Stack image provider support

Description

Add a limited Gcore Image Stack provider for the documented dimension and fit query controls, with additional mappings dependent on API verification.

Use Case

An application with Image Stack enabled needs responsive sizes from its existing Gcore delivery URLs.

Proposed Solution

• Add and export gcore through the core provider registry so Angular, React, and Svelte can use the shared configuration.
• Map verified width, height, and fit parameters, preserving the configured hostname, asset path, and existing URL data.
• Document Image Stack enablement and the initially supported fit modes. Obtain enabled-service fixtures to confirm the advertised resize behavior.
• Keep numeric quality and explicit output-format mappings out of the initial contract until documented and verified. Define how automatic service format negotiation is used without generating misleading explicit-format picture sources.
• Add setup documentation and focused fixtures covering dimensions, verified fit modes, query preservation, and automatic-format versus explicit-format behavior. Validate advertised dimensions and content types against an enabled service or controlled server where applicable.

Alternatives Considered

Use documented resize parameters manually or a custom query provider. A core adapter would cover the verified size/fit subset while leaving unsupported controls explicit.

Examples from Other Libraries

• Not found
• Image Stack