refactor: extract httpfetcher package from imgcache

Move HTTPFetcher, Config (was FetcherConfig), SSRF-safe dialer, rate
limiting, content-type validation, and related error vars from
internal/imgcache/fetcher.go into new internal/httpfetcher/ package.

The Fetcher interface and FetchResult type also move to httpfetcher
to avoid circular imports (imgcache imports httpfetcher, not the other
way around).

Renames to avoid stuttering:
  NewHTTPFetcher -> httpfetcher.New
  FetcherConfig  -> httpfetcher.Config
  NewMockFetcher -> httpfetcher.NewMock

The ServiceConfig.FetcherConfig field is retained (it describes what
kind of config it holds, not a stutter).

Pure refactor - no behavior changes. Unit tests for the httpfetcher
package are included.

refs #39

internal/imgcache/cache.go

@@ -9,6 +9,8 @@ import (
 	"io"
 	"path/filepath"
 	"time"
+
+	"sneak.berlin/go/pixa/internal/httpfetcher"
 )
 
 // Cache errors.
@@ -111,7 +113,7 @@ func (c *Cache) StoreSource(
 	ctx context.Context,
 	req *ImageRequest,
 	content io.Reader,
-	result *FetchResult,
+	result *httpfetcher.FetchResult,
 ) (ContentHash, error) {
 	// Store content
 	contentHash, size, err := c.srcContent.Store(content)