[enhancement] Add parameter with initial values for RamModel

### What's hard to do? (limit 100 words)

It is impossible to initialize [RamModel](https://github.com/google/xls/blob/d02431ca1247408f7e10d99b242d18cc7265b98d/xls/examples/ram.x#L165) with a given values. The need for this functionality was expressed as a [TODO](https://github.com/google/xls/blob/d02431ca1247408f7e10d99b242d18cc7265b98d/xls/examples/ram.x#L157) left in the code. Now, it should be possible to extend the module with an additional parameter

### Current best alternative workaround (limit 100 words)

Fill the RAM directly in the DSLX test

### Your view of the "best case XLS enhancement" (limit 100 words)

Add a parameter that accepts an array of initial values for `RamModel` to simplify RAM usage in tests