Feat: Build secrets

[dleviminzi]

Resolve BE-2149

This PR adds support for referencing secrets during builds.

First, create a secret.

beta9 secret create TEST TEST

Then, use with_secrets to flag that you would like it to be available during the build.

from beta9 import Image, endpoint

image = (
    Image()
    .with_secrets(["TEST"])
    .add_commands(
        [
            "echo $TEST",
        ]
    )
)


@endpoint(image=image, name="secret-test")
def handler():
    return "pass"

The build logs will look like this:

=> Building image 
Waiting for build container to start...
Python 3.10.16
TEST
...