Align sandbox exec arguments with subprocess

[pawalt]

Describe your changes

This commit adds new sandbox exec arguments, allowing the user to specify if they want to consume a stream directly, ignore the result, or consume it via a pipe.

Resolves WRK-422
Resolves WRK-413

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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I've tested this behavior out locally:

import modal
from modal.io_streams import StreamType

app = modal.App("peyton-sb-test")

@app.local_entrypoint()
async def main():
    sandbox = modal.Sandbox.create(
        timeout=1000,
        app=app,
    )

    exc = sandbox.exec("echo", "hello", stdout=StreamType.PIPE)
    print(exc.stdout.read(), end="")

    exc = sandbox.exec("echo", "hi2", stdout=StreamType.DEVNULL)
    try:
        print(exc.stdout.read())
    except modal.exception.InvalidError as e:
        print(e)

    exc = sandbox.exec("echo", "hi3", stdout=StreamType.STDOUT)
    exc.wait()
    try:
        print(exc.stdout.read())
    except modal.exception.InvalidError as e:
        print(e)

    sandbox.terminate()

Changelog

Output from Sandbox.exec can now be directed to /dev/null, stdout, or stored for consumption. This behavior can be controlled via the new StreamType arguments.

[freider]

Some minor comments and major realizations :D

[freider]

lgtm! didn't look to closely at the internals, but the public API makes sense to me! wdyt @mwaskom ?

[pawalt]

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