Add new refresh checksum cmd

[ligangty]

This command will refresh the checksums for a maven artifact in Ronda
maven repository.

[coveralls]

Pull Request Test Coverage Report for Build 8982286762

Details

• 54 of 123 (43.9%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 66.447%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
charon/cmd/init.py	0	3	0.0%
charon/pkgs/checksum_http.py	42	47	89.36%
charon/utils/files.py	7	14	50.0%
charon/cmd/cmd_checksum.py	0	54	0.0%

Totals
Change from base Build 8968398611:	0.2%
Covered Lines:	1915
Relevant Lines:	2882

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💛 - Coveralls