Add BAO_HEADERS env variable support

Is your feature request related to a problem? Please describe. I have my OpenBao instance behind HTTP IAP. I use header with IAP auth token to bypass IAP and reach openbao instance. So I am forced to supply -header argument to each of my cli calls. I would like to set this configuration parameter one time and apply it automatically to each of my CLI call in terminal session.

Describe the solution you'd like I would like to see BAO_HEADERS env var. Similar to vault cli https://github.com/hashicorp/vault/pull/21993

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

Explain any additional use-cases

Additional context