feat: support configurable AMD SEV-SNP KDS collateral endpoints

[WimYedema]

Allow operators to configure an AMD KDS-compatible mirror or cache during installation:

dstackup install --platform amd-sev-snp --sev-snp-kds-url "https://mirror.example.com/vcek/v1"

The configured endpoint is used by both:

• the KMS CVM via [core.attestation.urls].amd_kds;
• application CVMs via collateral_urls.amd_kds.

Motivation

AMD SEV-SNP key release can fail when the public AMD KDS endpoint is unavailable due to maintenance, outage, network restrictions, or disconnected deployment.

Previously, operators could not configure an alternative endpoint through dstackup install, so attestation collateral retrieval depended on the built-in public URL.

Behavior

• If --sev-snp-kds-url is provided, both KMS and application CVMs use that endpoint.
• If it is omitted, existing default AMD KDS behavior is unchanged.
• The endpoint must implement the AMD KDS API, including certificate-chain and chip/TCB-specific VCEK requests.
• dstack continues validating AMD signatures; configuring a mirror does not bypass attestation verification.

Related Use Cases

This supports:

• local pre-populated certificate caches;
• private enterprise mirrors;
• restricted networks;
• public KDS maintenance or outage recovery;
• fully offline AMD SEV-SNP deployments.