Add --https flag for e2e tests

[gouthamhusky]

This change introduces a --https flag for e2e tests

part of #1520

Changes:

• Add test/e2e/e2e_flags.go with --https flag infrastructure
• Update domain_mapping_test.go to respect --https flag when determining expected URL schemes
• Update test/README.md with documentation for the new flag

Testing:

• Flag parsing works correctly
• Tests compile without errors
• Flag is passed through test script properly
• Documentation updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: gouthamhusky / name: Goutham K (7d093f8, 830c07a, 9e7320f, a8af4a4, b09be45)

[knative-prow]

Welcome @gouthamhusky! It looks like this is your first PR to knative/client 🎉

[knative-prow]

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[knative-prow]

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