Fix #108: Implement client_credentials auth

[FlipWarthog]

Fix #108: Implement cient_credentials auth

@Crim Here's the minimal approach since there is a lot in common between grant_type=password and grant_type=client_credentials. Let me know if it's valid or you'd like it more separated from the existing logic.

[Crim]

I think what you have works fine, although my personal opinion would be to have an explicit/new SessionRefreshHandler implementation for this as well as a new XyzLoginCredentials class with only the required properties.

My thought process being, if any changes in the future need to happen to either login type, its easier/clearer to make and scoped to that login type specifically without any possibility of affecting the others.

If you're interested/have the time I'd prefer that approach, but if you're busy and don't have the time I understand and I can tinker with it.

Definitely appreciate the contribution either way!

[FlipWarthog]

Makes perfect sense and I can get those changes together.

[FlipWarthog]

Pushed those changes. Let me know if that's what you were looking for.

On Monday, when I'm back on my client laptop, I can smoke test this against their sandbox Pardot environment.

[FlipWarthog]

On Monday, when I'm back on my client laptop, I can smoke test this against their sandbox Pardot environment.

Confirmed working:

Executing request POST https://[client].sandbox.my.salesforce.com/services/oauth2/token HTTP/1.1 with [grant_type=client_credentials, client_id=[our client ID], client_secret=XXXXXXX]

Successfully gets an auth token.

[Crim]

Awesome thanks! I'll work on getting this merged in shortly! Appreciate the contribution!