`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

从 rsconnect/main.py 中的 add 命令开始，然后通过 rsconnect/metadata.py 跟踪 server_store.set，尤其是 _set 和 save。使用 .der 证书重现该命令，并检查证书值是如何存储的。当 rsconnect add 完成时不再出现 JSON 序列化错误，并且保存的 server 可以被 deploy 和 details 使用时，即表示完成。