[gitlab] Get secrets from vault

[nhulston]

What does this PR do?

Migrates secret storage from AWS SSM to Vault.
Secrets: https://vault.us1.ddbuild.io/ui/vault/secrets/kv/kv/k8s%2Fgitlab-runner%2Fdatadog-lambda-js%2Fsecrets/details?version=2

Motivation

Proper secret storage in Gitlab: https://datadoghq.atlassian.net/wiki/spaces/SECENG/pages/3826647944/Secure+Storage+and+Handling+of+Secrets#In-GitLab

Testing Guidelines

• Integration tests passed, so the vault kv get -field=dd-api-key works
• I published to a random region in sandbox successfully, so the vault kv get -field=$EXTERNAL_ID_NAME works. https://gitlab.ddbuild.io/DataDog/datadog-lambda-js/-/jobs/807550522

Additional Notes

Related PR: https://github.com/DataDog/vault-config/pull/7119

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)