Support converting enums from string values

Currently, when setting an Enum value on say a setting class, it is necessary to be explicit about it, i.e. if setting this: https://cakebuild.net/api/Cake.Common.Tools.SignTool/SignToolSignSettings/BE7882E3 you would need to do:

```
DigestAlgorithm = SignToolDigestAlgorithm.Sha256
```

If might be nice to be able to do something like this:

```
DigestAlgorithm = "sha256"
```

Where Cake would attempt to "find" an enum value that matched the string value, and if it can, set it appropriately.

@cake-build/cake-team what are your thoughts?

Start with the SignToolSignSettings.DigestAlgorithm API entry point and the SignToolDigestAlgorithm enum named in the issue. Clarify how string matching should work and where conversion applies, then verify that a value such as "sha256" can be accepted and mapped appropriately without changing explicit enum assignment.

Written by the indexing model from the issue text.