codecs: add base64 URL encoding without paddin

[KannarFr]

What

Adds serializeBase64UrlNoPadding() methods to embed biscuit tokens in URL parameters without padding characters safely (=).

Why

Padding characters in base64-encoded tokens can cause issues when used in URLs. This was mentioned in code comments about needing custom encoders to prevent Java's default padding in URL contexts.

How

• Added serializeBase64UrlNoPadding() to Biscuit and UnverifiedBiscuit classes
• Uses Java's built-in Base64.getUrlEncoder().withoutPadding()
• Existing fromBase64Url() already handles both formats automatically
• Updated docs to clarify that padded/unpadded strings are both accepted

Example

String token = biscuit.serializeBase64UrlNoPadding();
String url = "https://api.example.com/auth?token=" + token;
// No special encoding needed - works directly in URLs

Fully backward compatible, no breaking changes.