* Add base64 validation rule

Adds a base64 rule validating RFC 4648 strict encoding, a common
format check offered by other validation libraries (e.g. Zod's
.base64()) but missing from Laravel.

* Update src/Illuminate/Validation/Concerns/ValidatesAttributes.php

thx @shaedrich

Co-authored-by: Sebastian Hädrich <11225821+shaedrich@users.noreply.github.com>

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Co-authored-by: Sebastian Hädrich <11225821+shaedrich@users.noreply.github.com>