feat: Support envd build from stdin

[VoVAllen]

Description

In flyte, they use syntax like

@task(
        config=EnvdConfig(
        os="ubuntu20.04",
        pip_packages=["tensorflow"],
        language="python",
)

to configure the envd dependency. Then it needs to call envd build. it's better for envd build to support stdin input, which makes it easier for flyte to call envd

---

Message from the maintainers:

Love this enhancement proposal? Give it a 👍. We prioritise the proposals with the most 👍.

[cutecutecat]

What about syntax like:

@task(
        config=EnvdConfig(
                script="path/to/*.envd/dir",
                tag="train:latest",
                func="filename:func"
        )
)

If users straightly give parameters to the stdin of build, without a entry script, they might be inaccessible to most envd features, like expose, or we would design a huge EnvdConfig.