Support access keys for AWS Bedrock for Github deployments that cannot support OIDC

Currently the only way to configure this action to use AWS Bedrock is via OIDC for authentication. However, for self-hosted Github Enterprise deployments in high-security environments, there is no way to set up OIDC. IAM is a global service, and it requires the OIDC endpoints to be exposed to the internet, so environments that cannot expose their Github environment to the internet at all are unable to use this action, despite being able to reach Bedrock from within an internal network.

I think supporting some other authentication method like access keys would be really useful to allow us to use this tool in a high-security environment.