feat: Context.BasicAuth() convenience method

[FumingPower3925]

Summary

Add Context.BasicAuth() (username, password string, ok bool) that parses the Authorization: Basic <base64> header.

Motivation

While users can already do c.Header("authorization") + manual base64 decode, this is a common enough pattern that a convenience method improves DX. Gin, Echo, and the standard library all provide this.

Scope

• Parse authorization header for Basic scheme.
• Base64-decode and split on :.
• Return (username, password, true) or ("", "", false) if not present/invalid.
• Implementation: ~10 lines, mirrors net/http.Request.BasicAuth().

Non-breaking

Additive method on Context. No existing API changes required.

[FumingPower3925]

Implemented in v1.0.0. Closing as part of final review.