feat: implements conditional reads for GetObject and HeadObject

Closes #882

Implements conditional reads for `GetObject` and `HeadObject` in the gateway for both POSIX and Azure backends. The behavior is controlled by the `If-Match`, `If-None-Match`, `If-Modified-Since`, and `If-Unmodified-Since` request headers, where the first two perform ETag comparisons and the latter two compare against the object’s `LastModified` date. No validation is performed for invalid ETags or malformed date formats, and precondition date headers are expected to follow RFC1123; otherwise, they are ignored.

The Integration tests cover all possible combinations of conditional headers, ensuring the feature is 100% AWS S3–compatible.

s3api/utils/utils.go

@@ -642,6 +642,59 @@ func ParseCreateMpChecksumHeaders(ctx *fiber.Ctx) (types.ChecksumAlgorithm, type
 	return algo, chType, nil
 }
 
+// ConditionalHeaders holds the conditional header values
+type ConditionalHeaders struct {
+	IfMatch       *string
+	IfNoneMatch   *string
+	IfModSince    *time.Time
+	IfUnmodeSince *time.Time
+}
+
+// ParsePreconditionHeaders parses the precondition headers:
+// - If-Match
+// - If-None-Match
+// - If-Modified-Since
+// - If-Unmodified-Since
+func ParsePreconditionHeaders(ctx *fiber.Ctx) ConditionalHeaders {
+	ifMatch, ifNoneMatch := ParsePreconditionMatchHeaders(ctx)
+	ifModSince, ifUnmodeSince := ParsePreconditionDateHeaders(ctx)
+
+	return ConditionalHeaders{
+		IfMatch:       ifMatch,
+		IfNoneMatch:   ifNoneMatch,
+		IfModSince:    ifModSince,
+		IfUnmodeSince: ifUnmodeSince,
+	}
+}
+
+// ParsePreconditionMatchHeaders extracts "If-Match" and "If-None-Match" headers from fiber Ctx
+func ParsePreconditionMatchHeaders(ctx *fiber.Ctx) (*string, *string) {
+	return GetStringPtr(ctx.Get("If-Match")), GetStringPtr(ctx.Get("If-None-Match"))
+}
+
+// ParsePreconditionDateHeaders parses the "If-Modified-Since" and "If-Unmodified-Since"
+// headers from fiber context to *time.Time
+func ParsePreconditionDateHeaders(ctx *fiber.Ctx) (*time.Time, *time.Time) {
+	ifModSince := ctx.Get("If-Modified-Since")
+	ifUnmodSince := ctx.Get("If-Unmodified-Since")
+
+	var ifModSinceParsed, ifUnmodSinceParsed *time.Time
+
+	// the time format should be a valid RFC1123
+	// if parsing fails, ignore the error and leave the value as nil
+	modParsed, err := time.Parse(time.RFC1123, ifModSince)
+	if err == nil {
+		ifModSinceParsed = &modParsed
+	}
+
+	unmodParsed, err := time.Parse(time.RFC1123, ifUnmodSince)
+	if err == nil {
+		ifUnmodSinceParsed = &unmodParsed
+	}
+
+	return ifModSinceParsed, ifUnmodSinceParsed
+}
+
 // TagLimit specifies the allowed tag count in a tag set
 type TagLimit int