eba: support decoding MAC key provided in base64url format

eba: support decoding MAC key provided in base64url format

According to RFC 8555:

> The MAC key SHOULD be provided in base64url-encoded form...

However, currently we are only decoding the MAC key as base64.
This patch chooses the correct function to decode the user provided
MAC key. This can fix authentication error when a user uses command
`pvenode acme account register` and paste the EBA MAC key as prompted.

Signed-off-by: YU Jincheng <shana@zju.edu.cn>
Tested-by: Folke Gleumes <f.gleumes@proxmox.com>
 [ TL: fix typo in message, reword subject ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>