core: provide optional curl downloader.

[abiosoft]

This PR adds support for the specifying the downloader and expanding it to support curl. Resolves #1518.

The --downloader flag and the COLIMA_DOWNLOADER environment variable have been added.

Supported value are:

• native - Go's std http implementation (default)
• curl - the curl cli tool.

Usage

The flag takes precedence over the environment variable.

# use curl via flag
colima start --downloader=curl

# use curl via environment variable
COLIMA_DOWNLOADER=curl colima start