Adam Letts

[DA-253] storage api methods with asset service and local filesystem backends

2

This adds an abstract StorageBackend class with AssetServiceBackend and LocalFileBackend. One backend can be chosen as the primary for reads, while any number of backends can be chosen for writes....

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### 🔎 Search before asking | 提交之前请先搜索 - [x] I have searched the MinerU [Readme](https://github.com/opendatalab/MinerU) and found no similar bug report. - [x] I have searched the MinerU [Issues](https://github.com/opendatalab/MinerU/issues) and...