Base64 URL Safe Implementation

[microalps]

Base64 filters were added in Liquid 5.1 together with URL safe variants. There seems to be two variants out in the wild, and the question is can/should Liquid support both? base64_url_safe_decode supports both. base64_url_safe_encode does not.

1. Ruby's implementation swaps + and / only
2. Microsoft's implementations (Microsoft.IdentityModel.Tokens, System.Buffers.Text of .NET9) also remove the = padding.

RFC 4648

The pad character "=" is typically percent-encoded when used in an URI, but if the data length is known implicitly, this can be avoided by skipping the padding

Mozilla defines it as

A common variant is "Base64 URL safe", which omits the padding and replaces +/

Related to dotliquid#542

[microalps]

@dinsley @gmalette Do you have any insight on this?

[gmalette]

I don't have insight on this