✨ clusterctl: Add automatic selection of repository backend based on URL

[Arvinderpal]

What this PR does / why we need it:

This PR enables local file system repositories to be included in clusterctl. It identifies a repo as being local by seeing if its URL begins with "file://"

This is a follow up to: #1963

A sample clusterctl config with a local repo may look like this:

$ cat ~/.cluster-api/clusterctl.yaml 
providers:
- name: infra-foo
  url: file:///home/awander/.cluster-api/infra-foo/v0.1.0/infrastructure-components.yaml
  type: InfrastructureProvider

Which issue(s) this PR fixes
Rif: #1729

/assign @fabriziopandini
/cc @ncdc @vincepri

[k8s-ci-robot]

Hi @Arvinderpal. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[fabriziopandini]

/ok-to-test

[ncdc]

Thanks for adding the tests.

[ncdc]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Arvinderpal, ncdc

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