`rsconnect add server` with --cacert doesn't work with `.der` certs

Found while testing the --cacert fix, the .der cert never worked right with rsconnect add. We can run deploy and details with the .der certs but cannot rsconnect add.

We added .der support here: https://github.com/rstudio/rsconnect-python/pull/336

> rsconnect add -s https://alwayssecure.rsc:3443 -k {API_KEY} --cacert ~/Downloads/alwayssecure.rsc.der -n alwayssecureder
Detected the following inputs:
    name: COMMANDLINE
    server: COMMANDLINE
    api_key: COMMANDLINE
    insecure: DEFAULT
    cacert: COMMANDLINE
Checking https://alwayssecure.rsc:3443...        [OK]
Checking API key...                              [OK]
Traceback (most recent call last):
  File "/Users/kgartland/.pyenv/versions/rsconnect-tag/bin/rsconnect", line 8, in <module>
    sys.exit(cli())
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/rsconnect/main.py", line 169, in wrapper
    return func(*args, **kwargs)
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/rsconnect/main.py", line 463, in add
    server_store.set(
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/rsconnect/metadata.py", line 303, in set
    self._set(name, {**common_data, **target_data})
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/rsconnect/metadata.py", line 155, in _set
    self.save()
  File "/Users/kgartland/.pyenv/versions/3.8.2/envs/rsconnect-tag/lib/python3.8/site-packages/rsconnect/metadata.py", line 207, in save
    data = json.dumps(self._data, indent=4).encode("utf-8")
  File "/Users/kgartland/.pyenv/versions/3.8.2/lib/python3.8/json/__init__.py", line 234, in dumps
    return cls(
  File "/Users/kgartland/.pyenv/versions/3.8.2/lib/python3.8/json/encoder.py", line 201, in encode
    chunks = list(chunks)
  File "/Users/kgartland/.pyenv/versions/3.8.2/lib/python3.8/json/encoder.py", line 431, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/Users/kgartland/.pyenv/versions/3.8.2/lib/python3.8/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/Users/kgartland/.pyenv/versions/3.8.2/lib/python3.8/json/encoder.py", line 405, in _iterencode_dict
    yield from chunks
  File "/Users/kgartland/.pyenv/versions/3.8.2/lib/python3.8/json/encoder.py", line 438, in _iterencode
    o = _default(o)
  File "/Users/kgartland/.pyenv/versions/3.8.2/lib/python3.8/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type bytes is not JSON serializable

Start with the add command in rsconnect/main.py, then follow server_store.set through rsconnect/metadata.py, especially _set and save. Reproduce the command using a .der certificate and inspect how the certificate value is stored. Done means rsconnect add completes without the JSON serialization error and the saved server can be used by deploy and details.