api gateway: make conformance test target

[sarahalsmiller]

Changes proposed in this PR:

• Add two helper make targets to run api-gateway-conformance tests locally and clean up created files and resources afterwards

How I've tested this PR:

• run run-api-gateway-conformance
• run cleanup-api-gateway-conformance

How I expect reviewers to test this PR:

• run run-api-gateway-conformance
• run cleanup-api-gateway-conformance

Checklist:

• [ ] Tests added
• [ ] CHANGELOG entry added

  HashiCorp engineers only, community PRs should not add a changelog entry. Entries should use present tense (e.g. Add support for...)