Fix base64 encoding

[heifner]

Remove the invalid addition of "=" to all base64 encoded strings from fc::variant.
Also update base64 implementation from: https://github.com/ReneNyffenegger/cpp-base64

Resolves #1461

[spoonincode]

I know we ruled out boost's base64 impl, but can we use openssl's base64 instead of having our own impl?

[heifner]

I know we ruled out boost's base64 impl, but can we use openssl's base64 instead of having our own impl?

Doesn't seem to provide base64url_decode url decoding used by webauthn.

bummer

[linh2931]

I compared with the upstream version. It looked good. Approve the PR if we decide use this.