Implement DESIGN_MATRIX config keyword

[xjules]

The main of this is to specify an excel sheet (.xlsl) file containing the sensitivity analysis parameters. The argument list for DESIGN_MATRIX should include

• the path (to the excel sheet file)
• name of the sheet with the actual design parameters (design-sheet)
• name of the sheet with the default values (default-sheet)

An initial work has been already done by @oyvindeide https://github.com/equinor/ert/pull/8099

This epic should include the following features too:

• [x] #8689
• [ ] https://github.com/equinor/ert/issues/8961
• [ ] https://github.com/equinor/ert/issues/8686
• [ ] https://github.com/equinor/ert/issues/8685
• [ ] https://github.com/equinor/ert/issues/8784