Add `b''` form to represent bytes

[p1g3]

I noticed that cel-go supports the b'' format for representing bytes in expressions.

I noticed that cel-go supports the b'' format for representing bytes in expressions. Why doesn't expr support the b'' form for representing bytes? Is there an alternative way to represent bytes?

https://github.com/google/cel-spec/blob/master/doc/langdef.md#string-and-bytes-values

[antonmedv]

I can add such feature. Now just use strings for bytes.