Implement "ignore_metadata" option in openEO reducers for the WCPS backend

[pierocampa]

The correct implementation of openEO reducers with respect to nils/nodata handling is fundamental.

The reducers involved are:

• [ ] all
• [ ] any
• [ ] first
• [ ] last
• [ ] max
• [ ] mean
• [ ] median
• [ ] min
• [ ] product
• [ ] sd
• [ ] sum
• [ ] variance

Note: the quantiles process is not available for the WCPS backend natively (there is the sort operator since v10.1.0 which could be used, but nothing more than that), so the median reducer might be ignored for now.