Add Mac support to credentials plugin

[lreimer]

Add support for the Mac OS keychain.

[lreimer]

Need to do some major refactoring on how the credentials are represented and accessed in the plugin.

Also need to build a common API for both Windows and Mac, both are quite different.

[lreimer]

Initial development is done. Will do some integration testing now and then publish the plugin to Bintray and Gradle plugin repo.

[lreimer]

Integration testing went OK. Still, on MacOS a few minor issues occurred when accessing the default keychain from different processes.

So I will add a new configuration convention where you can set the actual keychain or security file location, something like

credentials {
   keychain = '/Users/seu-as-code/Library/Keychain/seu-as-code.keychain'
}

[lreimer]

May need to interpolate the keychain string with environment variables or system properties to make things more convenient. But this can be handled by standard Groovy functionality I think.

[lreimer]

So far initial integration testing looks good. Will wait a little longer and than release final version of the plugin.

[ghost]

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[ghost]

Thank you, everything looks good now! :bowtie: