rust/base64: add decoder

rust/base64: add decoder

Add a pure rust base64 decoder. This supports 3 modes of operation just
like the C decoder as follows.
1. RFC 2045
2. RFC 4648
3. Strict

One notable change is that "strict" mode is carried out by the rust
base64 crate instead of native Rust. This crate was already used for
encoding in a few places like datasets of string type. As a part of this
mode, now, only the strings that can be reliably converted back are
decoded.

The decoder fn is available to C via FFI.

Bug 6280
Ticket 7065
Ticket 7058