br support http download

[zhouqiang-cl]

Feature Request

Describe your feature request related problem:

Now we just support local or s3 to backup. but local eat too much disk and s3 is expensive. I think we can support download from HTTP

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[kennytm]

Note that in the current design, an external storage cannot "just support download". It must define all these 3 operations:

1. Download a file
2. Upload a file
3. Checks if a file exists

An HTTP storage clearly supports operation 1 and 3 via GET and HEAD requests. The question is what should operation 2 do.

(In CrDB's BACKUP statement, it will issue a PUT request with the body being the file.)

Additionally, if we want to support Lightning, we will need this additional operation which definitely isn't supported by HTTP:

4. Recursively list content of directory