pkg/verify: Refactor signature fetching to only use a Store interface

[wking]

This abstracts signature retrieval, setting the stage for future pivots such as fetching signatures from config maps in the local cluster.

Spun off from these suggestions for #279. I've left the file implementation in place to minimize the diff (in 0cc3e41 I'd replaced it with an in-memory sig store). I've also left both the file and HTTP stores in the verify.go file to minimize the diff (in 0cc3e41 I'd moved the HTTP store into a subpackage to simplify verify.go).

[wking]

e2e-aws had:

fail [github.com/openshift/origin/test/extended/prometheus/prometheus_builds.go:163]: Expected
    <map[string]error | len:1>: {
        "ALERTS{alertname!~\"Watchdog|AlertmanagerReceiversNotConfigured\",alertstate=\"firing\"} >= 1": {
            s: "promQL query: ALERTS{alertname!~\"Watchdog|AlertmanagerReceiversNotConfigured\",alertstate=\"firing\"} >= 1 had reported incorrect results:\n[{\"metric\":{\"__name__\":\"ALERTS\",\"alertname\":\"FailingOperator\",\"alertstate\":\"firing\",\"endpoint\":\"https-metrics\",\"exported_namespace\":\"e2e-test-olm-23440-mnls8\",\"instance\":\"10.130.0.17:8081\",\"job\":\"olm-operator-metrics\",\"name\":\"etcdoperator.v0.9.4\",\"namespace\":\"openshift-operator-lifecycle-manager\",\"phase\":\"Failed\",\"pod\":\"olm-operator-684dcf9457-dzgjx\",\"reason\":\"NoOperatorGroup\",\"service\":\"olm-operator-metrics\",\"severity\":\"info\",\"version\":\"0.9.4\"},\"value\":[1578446043.22,\"1\"]}]",
        },
    }
to be empty

/retest

[jottofar]

/cc @crawford PTAL

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/cc @crawford PTAL

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[wking]

Rebased around #279 with 2bd263b -> 921f3ea, laying the groundwork for using this to address rhbz#1840343 (parallel HTTP(S) signature fetching).

[openshift-ci-robot]

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