Crash when using remote execution on a large element

Trying to build a large element (and after raising limit on the number of open files to work around https://github.com/apache/buildstream/issues/1842), I got another crash

```
An unhandled exception occured:

Traceback (most recent call last):
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/_scheduler/jobs/job.py", line 346, in child_action
result = self.child_process() # pylint: disable=assignment-from-no-return
^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 81, in child_process
return self._action_cb(self._element)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/_scheduler/queues/buildqueue.py", line 55, in _assemble_element
element._assemble()
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/element.py", line 1687, in _assemble
collect = self.assemble(sandbox) # pylint: disable=assignment-from-no-return
^^^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/buildelement.py", line 315, in assemble
with sandbox.batch(root_read_only=True, label="Running commands"):
File "/usr/lib/python3.11/contextlib.py", line 144, in __exit__
next(self.gen)
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/sandbox/sandbox.py", line 265, in batch
batch.execute()
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/sandbox/_sandboxreapi.py", line 234, in execute
self.sandbox._run_with_flags(
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/sandbox/sandbox.py", line 374, in _run_with_flags
return self._run(command, flags=flags, cwd=cwd, env=env)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/sandbox/_sandboxreapi.py", line 101, in _run
action_result = self._execute_action(action, flags) # pylint: disable=assignment-from-no-return
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/sandbox/_sandboxremote.py", line 302, in _execute_action
cascache.pull_tree(casremote, tree_digest)
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/_cas/cascache.py", line 251, in pull_tree
digest = self._fetch_tree(remote, digest)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/_cas/cascache.py", line 617, in _fetch_tree
dirdigests = self.add_objects(buffers=dirbuffers)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/buildstream/_cas/cascache.py", line 357, in add_objects
response = local_cas.CaptureFiles(request)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/grpc/_channel.py", line 1181, in __call__
return _end_unary_response_blocking(state, call, False, None)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/abderrahimkitouni/.local/pipx/venvs/buildstream/lib/python3.11/site-packages/grpc/_channel.py", line 1006, in _end_unary_response_blocking
raise _InactiveRpcError(state) # pytype: disable=not-instantiable
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
grpc._channel._InactiveRpcError: <_InactiveRpcError of RPC that terminated with:
status = StatusCode.RESOURCE_EXHAUSTED
details = "CLIENT: Received message larger than max (7309916 vs. 4194304)"
debug_error_string = "UNKNOWN:Error received from peer unix:/tmp/buildstreample8mkmu/cas/casserver-l6ur9wu9.sock {grpc_message:"CLIENT: Received message larger than max (7309916 vs. 4194304)", grpc_status:8, created_time:"2024-10-09T14:46:23.774345481+01:00"}"
>
```

This is again a problem with the current implementation of `CasCache.add_objects()`.

A see two ways to trackle this (and https://github.com/apache/buildstream/issues/1842):
* Use `BatchUpdateBlobs` to send the data to buildbox-casd, composing batches of appropriate size. Since all the data is already in memory.
* Use `root_directory_digest` instead of `tree_digest` from the output directories of an action result (we may need to set `output_directory_format` and servers are not required to support it)

Start with buildstream/_cas/cascache.py, especially CasCache.add_objects(), _fetch_tree(), and pull_tree(), then follow the remote execution path through sandbox/_sandboxremote.py. Reproduce a large-element remote build and inspect the gRPC message-size failure; done means the build completes without RESOURCE_EXHAUSTED for large directory data.

Written by the indexing model from the issue text.