Flux CLI change to add provider field to GitRepository spec.

- Add provider flag to `flux create source git` command with supported values: azure, generic.

- Unit tests validating the generated yaml and error conditions.

Signed-off-by: Dipti Pai <diptipai89@outlook.com>

cmd/flux/testdata/create_source_git/source-git-provider-azure.yaml

@@ -0,0 +1,12 @@
+---
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: podinfo
+  namespace: flux-system
+spec:
+  interval: 1m0s
+  provider: azure
+  ref:
+    branch: test
+  url: https://dev.azure.com/foo/bar/_git/podinfo

cmd/flux/testdata/create_source_git/source-git-provider-generic.yaml

@@ -0,0 +1,12 @@
+---
+apiVersion: source.toolkit.fluxcd.io/v1
+kind: GitRepository
+metadata:
+  name: podinfo
+  namespace: flux-system
+spec:
+  interval: 1m0s
+  provider: generic
+  ref:
+    branch: test
+  url: https://github.com/stefanprodan/podinfo