Remove DdApiKeySecretArn AllowedPattern constraints to allow friendly name support in DD forwarder lambda

[LorisFriedel]

What does this PR do?

To allow the use of the simple (aka friendly) name of the DD API key secret, I'm removing the AllowedPattern constraints on the DdApiKeySecretArn template variable and updating the variable description accordingly.

Motivation

This allows to use a naming convention for the secret names and deploy the lambda without having to know the full ARN of the secret. This makes scaling the use of the Datadog Forwarder much easier.

Testing Guidelines

The lambda code is using boto3.client.get_secret_value(SecretId=<var>), which, according to its documentation, allows to retrieve a secret from its ARN or its name (if the secret is in the same account):

SecretId (string) – [REQUIRED]
The ARN -->or name<-- of the secret to retrieve. To retrieve a secret from another account, you must use an ARN.

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
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