[FEAT] Support IAM / EC2 auth methods

**Is your feature request related to a problem? Please describe.**
I have private github runners in AWS that already have an instance profile. Calls to vault should be able to leverage that instance profile role or another assumed role to auth.

**Describe the solution you'd like**
An additional authentication method in this action code that supports vault AWS auth options.

**Describe alternatives you've considered**
Use sts assume role and vault cli commands to retrieve the secrets in a bash script.

**Additional context**
https://www.vaultproject.io/docs/auth/aws

Start by reviewing the action's existing authentication methods and the linked Vault AWS authentication documentation. Define support for authenticating with an EC2 instance profile or an assumed role, then verify that the GitHub Action can retrieve Vault secrets through the selected AWS auth option.

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