Handling BigDecimal-like vectors in the expression engine

[jarohen]

Vectors: DecimalVector, Decimal256Vector

• See Handling Arrow from external sources in the query engine #1961 for the main checklist
• You'll also need to consider how this fits into the types/arrow-type-hierarchy, and likely adapt a lot of the expr/codegen-call arithmetic operations to cope with (e.g.) adding a Float8 to a Decimal (including the type such an operation should return)
• This is definitely the trickier of the Handling Arrow from external sources in the query engine #1961 sub-tasks! (but I don't intend to create a 'good-second-issue' label)

[FiV0]

#2580 Partially addressed this card. We need to revisit this when expanding the expression engine (vararg least/greatest). Also addressing the full range of Arrow Decimal precision/scale should be possible.