Add container filesystem API for sandboxes

[azliu0]

Describe your changes

Resolves WRK-414 by adding a new container filesystem API.

This API is designed to closely follow io.FileIO, supporting most of the major functions (open, read, readline, readlines, write, seek, flush, close) along with a few higher-level functions (delete_bytes, write_replace_bytes) to help make Sandboxes a bit easier to use.

All file modes (e.g. text, bytes, r/w/a/x, etc.) are supported, as well as most common errors/exceptions. If an unknown exception is thrown, then a modal.exception.FilesystemExecutionError is thrown instead.

Example usage:

app = modal.App.lookup("sandbox-fs", create_if_missing=True)
sb = modal.Sandbox.create(app=app)

with sb.open("test.txt", "w") as f:
  f.write("Hello World\n")

f = sb.open("test.txt", "rb")
print(f.read())

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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Changelog

Sandboxes now support a new filesystem API. The open() method returns a FileIO handle for native file handling in sandboxes.

app = modal.App.lookup("sandbox-fs", create_if_missing=True)
sb = modal.Sandbox.create(app=app)

with sb.open("test.txt", "w") as f:
  f.write("Hello World\n")

f = sb.open("test.txt", "rb")
print(f.read())

[pawalt]

@azliu0 can you convert this to draft until we have worker & server support? Also this will need to be tested somehow. Need to mock the underlying RPCs.

[mwaskom]

Can we improve the changelog a bit here?