support SeaweedFS as storage backend

[ghost]

What would you like to be added:
support SeaweedFS as storage backend

Why is this needed:
SeaweedFS is very optimized for small files and it has a different protocal compared to s3.

[SandyXSD]

Any issue when you use it with the S3 protocol?

[ghost]

s3 of SeaweedFS requires setting up Filer, which works like an alternative of juicefs.

seeweedFS-like system will generate a key for the uploaded file instead of using the user providied key like s3.

SeaweedFS is (kind of) similar to the block device mode in juicefs for acceleratingsmall files , but SeaweedFS does not require shared block-device and it's better for a clould-native enviroment.

[davies]

It's very difficult to use SeaweedFS without the filer under current architecture, so we won't put it int he roadmap.