Binary encoder for numeric datatype

[Gustl22]

• Closes Problems with numeric data type stablekernel/postgresql-dart#164
• Binary Encoder for numeric / decimal SQL Data Type
• Fix decoding numeric values with trailing or leading zeros
• More tests

It seems that the implementation is quite heavy, but when I looked into other repositories, they also have a bulk of code:

• BigDecimal Implementation of a14n, which cannot be applied 1:1, as Postgres Numeric uses another binary format than BigDecimal.
• BigDecimal Implementation of Java: Same applies to Java.
• You may find another lib which implements the binary format, then let me know (e.g. node-postgres doesn't).

[isoos]

I think this looks good, but I'd like to see the RegExp values to be extracted as top-level fields. There is a chance to optimize the string concatenations a bit, but I don't mind the current state, it doesn't seem to have too much runtime benefit to do it right away.

[isoos]

I've resolved the merge conflicts in the CHANGELOG, thanks for doing this! I think combined with your other PR, this is worth to get released right away :)

[isoos]

Pubilshed as 2.4.0

[Gustl22]

@isoos Big thanks for reviewing contributions, and for maintaining and providing such projects!