Add `Equal()` helper method to `pcommon.Value`

[dmathieu]

          nit: I know that `AsRaw()` could make a copy of the data. This is a reminder that it would be nice if you could write `a.Value().Equal(value)` and avoid the allocation.

Originally posted by @jmacd in #12390 (comment)

So we don't need to use AsRaw to do value comparison.

[bogdandrutu]

The signature of the "AddAttribute" is not respecting our standard, pleas consider, either one of the options:

1. Rename AddAttributes to AddRawAttribute
2. Replace value any with value Value

[dmathieu]

Which one would you prefer?

[jmacd]

I prefer the second option, value Value.

[bogdandrutu]

I do also prefer the second option.

[bogdandrutu]

We still need the Equal but they will accept the same type.

[jmacd]

Wait a second -- the original comment of mine was to point out that Equal() could avoid allocations. If the user has to create a value object to perform the comparison, won't they have the same copy? The problem is that to construct a value w/ any of the complex types, you have to copy for safety, right (e.g., []byte values, slice values, map values).

[bogdandrutu]

@jmacd my comment was to change AddAttribute to accept Value and not any.