[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
```

首先，将 C# 库的 HierarchicalMerge 行为与 CLI 的层级合并和扁平合并方法进行比较，然后确定 Python 库中对应的公共 API。该 issue 提议 hierachical_merge(boms, metadata_component) 和 flat_merge(boms)；当两种策略都以与现有实现一致的行为对外提供，并为用户编写文档时，即视为完成。

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