**Apache Airflow version**: 2.0.0

**Environment**: 

- **Cloud provider or hardware configuration**: N/A
- **OS** (e.g. from /etc/os-release): custom Docker image (`FROM python:3.6`) and macOS Big Sur (11.0.1)
- **Kernel** (e.g. `uname -a`): 
  - `Linux xxx 4.14.174+ #1 SMP x86_64 GNU/Linux`
  - `Darwin xxx 20.1.0 Darwin Kernel Version 20.1.0 rRELEASE_X86_64 x86_64`
- **Install tools**:
- **Others**:

**What happened**:

I may have mixed 2 different issues here, but this is what happened to me.

I'm trying to use Airflow with the `airflow.providers.google.cloud.secrets.secret_manager.CloudSecretManagerBackend` and a `sql_alchemy_conn_secret` too, however, I have a `NameError` exception when attempting to run either `airflow scheduler` or `airflow webserver`:
```
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/airflow/__init__.py", line 34, in <module>
    from airflow import settings
  File "/usr/local/lib/python3.6/site-packages/airflow/settings.py", line 35, in <module>
    from airflow.configuration import AIRFLOW_HOME, WEBSERVER_CONFIG, conf  # NOQA F401
  File "/usr/local/lib/python3.6/site-packages/airflow/configuration.py", line 786, in <module>
    conf.read(AIRFLOW_CONFIG)
  File "/usr/local/lib/python3.6/site-packages/airflow/configuration.py", line 447, in read
    self._validate()
  File "/usr/local/lib/python3.6/site-packages/airflow/configuration.py", line 196, in _validate
    self._validate_config_dependencies()
  File "/usr/local/lib/python3.6/site-packages/airflow/configuration.py", line 224, in _validate_config_dependencies
    is_sqlite = "sqlite" in self.get('core', 'sql_alchemy_conn')
  File "/usr/local/lib/python3.6/site-packages/airflow/configuration.py", line 324, in get
    option = self._get_option_from_secrets(deprecated_key, deprecated_section, key, section)
  File "/usr/local/lib/python3.6/site-packages/airflow/configuration.py", line 342, in _get_option_from_secrets
    option = self._get_secret_option(section, key)
  File "/usr/local/lib/python3.6/site-packages/airflow/configuration.py", line 303, in _get_secret_option
    return _get_config_value_from_secret_backend(secrets_path)
NameError: name '_get_config_value_from_secret_backend' is not defined
```

**What you expected to happen**:

A proper import and configuration creation.

**How to reproduce it**:

`airflow.cfg`:
```ini
[core]

# ...

sql_alchemy_conn_secret = some-key

# ...

[secrets]

backend = airflow.providers.google.cloud.secrets.secret_manager.CloudSecretManagerBackend

backend_kwargs = { ... }

# ...

```

**Anything else we need to know**:

Here is the workaround I have for the moment, not sure it works all the way, and probably doesn't cover all edge cases, tho it kinda works for my setup:

Move `get_custom_secret_backend` before (for me it's actually below `_get_config_value_from_secret_backend`): https://github.com/apache/airflow/blob/cc87caa0ce0b31aa29df7bbe90bdcc2426d80ff1/airflow/configuration.py#L794

Then comment: https://github.com/apache/airflow/blob/cc87caa0ce0b31aa29df7bbe90bdcc2426d80ff1/airflow/configuration.py#L232-L236

airflow/configuration.py
val = str(self.get(section, key, **kwargs)).lower().strip()
        if '#' in val:
            val = val.split('#')[0].strip()
        if val in ('t', 'true', '1'):
            return True
        elif val in ('f', 'false', '0'):
            return False
        else:
            raise AirflowConfigException(
                f'Failed to convert value to bool. Please check "{key}" key in "{section}" section. '
                f'Current value: "{val}".'
            )
    def getint(self, section, key, **kwargs):
        val = self.get(section, key, **kwargs)
        try:
            return int(val)
        except ValueError:
            raise AirflowConfigException(
                f'Failed to convert value to int. Please check "{key}" key in "{section}" section. '
                f'Current value: "{val}".'
            )
    def getfloat(self, section, key, **kwargs):