DigestUtils can be mis-used to create a vulnerability

[magner669]

Problem:

Tracking issue for:

• https://github.com/Citi/gradle-helm-plugin/security/code-scanning/2

A methid in the class DigestUtilsl takes string with the digest algorithm name.

The default algorithm name is sha-256 which is secure. Howver user can pass in an unsecure algorithm such as sha-1, or md5, and this might be un noticed by the tools.

Proposed solution:

Refactor to take MesageDigest object rather than the algo name.
Set the default to sha-256 as before.
Refactor call points to build and pass in MessageDigest rather than string.

This should attribute a volnerabilty to call point, and allow us to close this issue. Other scan alterts might be opened for call points which should be much easier to remidiate.