[Jira]: Improve attachment download methods.

[jerubball]

Improved get_attachment_content() as following:

• Use content URL obtained from get_attachment() metadata.
• Use Accept: */* request header instead of the default of Accept: application/json.

Modified download_issue_attachments() as following:

• Check for existing file before sending download request, and add overwrite parameter to skip this check.
• Add stream parameter to more efficiently process large attachment zip file, and block_size parameter for control.
• Add timeout parameter to raise HTTPError to handle cases where download freezes.