Support custom headers to directory and authorizer

[ronenh]

This gives users the ability to configure topaz to include additional headers (gRPC metadata) in all directory calls. The remote_directory config value is alread an aserto.Config, which has a Headers field. This commit simply wires that into the underlying connection. Example:

remote_directory:
  address: directory.prod.aserto.com:8443
  api_key: <api key>
  tenant_id: <tenant ID>
  headers:
    header-1: value-1
    header-2: "value 2"

It also gives the same ability to the directory and authorizer CLI commands. For example:

topaz ds get manifest --headers "foo=bar;a=b"

or (same semantics):

topaz ds get manifest --headers "foo=bar" --headers "a=b"

[coveralls]

Pull Request Test Coverage Report for Build 10477947833

Details

• 3 of 14 (21.43%) changed or added relevant lines in 2 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 28.178%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/app/directory/simple_resolver.go	3	5	60.0%
plugins/edge/plugin.go	0	9	0.0%
<!--	Total:	3	14	21.43%	-->

Files with Coverage Reduction	New Missed Lines	%
plugins/edge/plugin.go	1	0.0%
pkg/cli/cc/ports.go	6	76.27%
<!--	Total:	7		-->

Totals
Change from base Build 10414409342:	-0.2%
Covered Lines:	1321
Relevant Lines:	4688

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💛 - Coveralls