Add method to fetch BearerToken from Header / Cookies

- `BearerToken` allow to parse header/cookies to fetch bearer token
- `headerAuth` allow to parse header to fetch bearer token
- `cookieAuth` allow to parse header to fetch bearer token

closes #2
closes #11

Signed-off-by: Evgeniy Kulikov <kim@nspcc.ru>

header.go

@@ -0,0 +1,76 @@
+package main
+
+import (
+	"bytes"
+	"encoding/base64"
+
+	"github.com/nspcc-dev/neofs-api-go/pkg/token"
+	"github.com/pkg/errors"
+	"github.com/valyala/fasthttp"
+)
+
+const bearerToken = "Bearer"
+
+// BearerToken usage:
+//
+// if tkn, err = BearerToken(c); err != nil && tkn == nil {
+// 	log.Error("could not fetch bearer token", zap.Error(err))
+// 	c.Error("could not fetch bearer token", fasthttp.StatusBadRequest)
+// 	return
+// }
+var _ = BearerToken
+
+func headerAuth(h *fasthttp.RequestHeader) (*token.BearerToken, error) {
+	auth := h.Peek(fasthttp.HeaderAuthorization)
+	if auth == nil || !bytes.Contains(auth, []byte(bearerToken)) {
+		return nil, nil
+	}
+
+	auth = bytes.ReplaceAll(auth, []byte(bearerToken+" "), nil)
+
+	data, err := base64.StdEncoding.DecodeString(string(auth))
+	if err != nil {
+		return nil, errors.Wrap(err, "could not fetch marshaled from base64")
+	}
+
+	tkn := new(token.BearerToken)
+	if err = tkn.Unmarshal(data); err != nil {
+		return nil, errors.Wrap(err, "could unmarshal bearer token")
+	}
+
+	return tkn, nil
+}
+
+func cookieAuth(h *fasthttp.RequestHeader) (*token.BearerToken, error) {
+	auth := h.Cookie(bearerToken)
+	if auth == nil {
+		return nil, nil
+	}
+
+	data, err := base64.StdEncoding.DecodeString(string(auth))
+	if err != nil {
+		return nil, errors.Wrap(err, "could not fetch marshaled from base64")
+	}
+
+	tkn := new(token.BearerToken)
+	if err = tkn.Unmarshal(data); err != nil {
+		return nil, errors.Wrap(err, "could unmarshal bearer token")
+	}
+
+	return tkn, nil
+}
+
+func BearerToken(ctx *fasthttp.RequestCtx) (*token.BearerToken, error) {
+	// ignore empty value
+	if ctx == nil {
+		return nil, nil
+	}
+
+	if tkn, err := headerAuth(&ctx.Request.Header); err != nil {
+		return nil, err
+	} else if tkn != nil {
+		return tkn, nil
+	}
+
+	return cookieAuth(&ctx.Request.Header)
+}