Add BigDecimal support for SpEl numeric operations

Prior to this change, SpEL supported numeric operations for int, float,
ect. `new java.math.BigDecimal('0.1') > 0` evaluated to false
(BigDecimal is truncated to int)

This commit introduces support for BigDecimal operations for all
mathematical operators. `new java.math.BigDecimal('0.1') > 0` now
evaluates to true (the comparison is made with BigDecimals)

Issue: SPR-9164
Merge pull request #80 from giovannidalloglio/SPR-9164

* SPR-9164:
  Add BigDecimal support for SpEl numeric operations