Title: jwt_authn: breaking change in payload encoding

Description:
The encoding format for the JWT Payload header in the jwt_authn filter was changed in #14779. This is a breaking change in golang due to the way the base64 package works: https://golang.org/pkg/encoding/base64/#pkg-variables

Repro steps:
see test changes here: https://github.com/envoyproxy/envoy/pull/14779/files#diff-ccb8c0f9dfe4011484c9d7ca83d56abbe8b60c7b1b8dcd8efbc3debe0c6c5a1cR179

Config:
n/a

Logs:
n/a

Call Stack:
n/a