This commit modifies the surrogate key storage mechanism to respect the
TTL (Time-To-Live) specified in the Caddyfile's `ttl` directive.

Previously, surrogate keys were stored with a hardcoded, often indefinite,
TTL based on the storage backend (e.g., Redis, Badger). This change
propagates the TTL from the main cache configuration down to the
surrogate key provider.

Key changes include:
- Modified `pkg/surrogate/providers/common.go`:
    - `baseStorage.init()` now accepts a `defaultTTL` parameter.
    - It uses this `defaultTTL` for surrogate key duration if positive;
      otherwise, it falls back to the previous `storageToInfiniteTTLMap`.
- Updated `pkg/surrogate/surrogate.go`, `pkg/surrogate/providers/factory.go`,
  and `pkg/surrogate/providers/souin.go` to pass the `defaultTTL`
  through their respective initialization functions.
- Updated `pkg/middleware/middleware.go`:
    - `NewHTTPCacheHandler` now retrieves the `defaultTTL` from the
      configuration and passes it to `surrogate.InitializeSurrogate`.
- Added tests in `pkg/surrogate/providers/common_test.go`:
    - Introduced a mock storer to capture the duration passed to `Set`.
    - New test cases verify that custom TTLs and default fallback
      TTLs are correctly applied to surrogate keys.

This resolves the issue where surrogate keys did not expire according to
the configured cache TTL, potentially leading to stale data and excessive
storage usage.

This commit addresses compilation errors introduced by the previous
changes for surrogate key TTL handling.

The errors included:
- Missing `time` package imports in provider files.
- Mismatched function signatures for provider generation functions
  (`generate...Instance`) and their calls in `SurrogateFactory`.
- Mismatched arguments for `baseStorage.init()` calls within
  specific providers (Akamai, Cloudflare, Fastly).

Changes made:
- Added `import "time"` to `pkg/surrogate/providers/factory.go`,
  `souin.go`, `akamai.go`, `cloudflare.go`, and `fastly.go`.
- Updated `generateAkamaiInstance`, `generateCloudflareInstance`, and
  `generateFastlyInstance` function signatures to accept the
  `defaultTTL time.Duration` parameter.
- Ensured that these provider generation functions correctly pass the
  `defaultTTL` to their respective embedded `baseStorage.init()` calls.

These changes should resolve the build failures and ensure the TTL
propagation logic compiles successfully.