Improve retry when download assets and prompt proxy suggestion

[tisonkun]

What type of enhancement is this?

Other

What does the enhancement do?

greptimedb/scripts/fetch-dashboard-assets.sh

Lines 23 to 29 in e481f07

	# Download the SHA256 checksum attached to the release. To verify the integrity
	# of the download, this checksum will be used to check the download tar file
	# containing the built dashboard assets.
	curl -Ls ${GITHUB_URL}/GreptimeTeam/dashboard/releases/download/$RELEASE_VERSION/sha256.txt --output sha256.txt
	# Download the tar file containing the built dashboard assets.
	curl -L ${GITHUB_URL}/GreptimeTeam/dashboard/releases/download/$RELEASE_VERSION/build.tar.gz --output build.tar.gz

Currently, we use the default option of curl to download dashboard assets. It's possible suffering network issue and timeout after a relatively long time.

We'd better improve the retry strategy (e.g., retry times, shorter retry interval) and prompt the suggestion to use GHPROXY if users do not configure it (or http_proxy that curl would respect, I don't determinate a good suggestion prompt here).

Implementation challenges

If we keep using shell (bash), we can add a function to do the retry. Or even we can rewrite the script with Python so it's more programmatic