Set a timeout for outgoing HTTP requests

[jwodder]

A timeout should be set (via reqwest::ClientBuilder::timeout()) on reqwest clients for use when dandidav makes an outgoing HTTP request.

• Note that this will only affect requests made to Dandi Archive, to datasets.datalad.org, and to S3 when determining a bucket's region. Timeouts for requests made via the S3 SDK will have to be configured separately (assuming they're not already set to something reasonable).

• For comparison:

  • The only timeout dandi-cli uses is a 30-second timeout when performing file downloads.
  • httpx's default timeout is 5 seconds.

• Errors raised as a result of such timeouts should be converted to 504 responses.