add S3_READ_TIMEOUT environment variable

[tateisu]

When using tootctl media remove or media upload from WebUI or client,
I saw many Net::ReadTimeout with #<TCPSocket:(closed)> .
5s default value is too short for cheap storage service.
This PR add S3_READ_TIMEOUT environment variable to configure read timeout.

See also: #12459

[mashirozx]

Suggestion: should we consider different max timeout with different file sizes?

For example we should wait 20 seconds to upload a video but shouldn't wait more than 5 secon for a image.

[tateisu]

I don't think the timeout value should be determined by the file size.
It's an unnecessary optimization, It does NOT optimize transfer speed in case of successful upload.

I just want set enough timeout value that might not happen in successful case.