[FEAT] Define hierarchical and flat merge methods

The C# Library and the CLI tool expose to the user the methods for merging different SBOMs using either hierarchical or flat strategies. This is completely absent in the python library and forces to user to write them leading to differences in implementations. Would it make sense to expose this function/method? The C# library HierarchicalMerge function takes in a list of boms and the merge component, so i guess it would be something like

```python
def hierachical_merge(boms, metadata_component):
# merge stuff
return merged_bom
```
while the flat merge

```python
def flat_merge(boms):
return merged_bom
```