Enhance and fix libsecret credential store to become default on Linux

[mjcheetham]

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The configuration of a credential store on Linux is still a manual process as outlined by https://aka.ms/gcmcore-linuxcredstores.

We may want to look at introducing either some smart detection or configuration of one of the more secure options (secretservice or gpg) depending on environment, or even make the configure command prompt the user to select.

[vtbassmatt]

Sketch of the solution:

1. Try to probe for secretservice and/or gpg.
2. For any of the above which are available, plus the others which are potentially insecure but always available, ask the user during configure time which one they want to set up.