Create a new Secret parameter or as keyword

Add support for secret parameter types, similar to Pydantic's [SecretStr](https://docs.pydantic.dev/2.0/usage/types/secrets/)/SecretBytes implementation, to help protect sensitive information from being accidentally exposed in logs, debugging output, or string representations.

I imagine either having a dedicated param:
```python
import param

class Config(param.Parameterized):
api_key = param.Secret(default="sk_test_abcdef")

config = Config()
print(config.api_key) # Would display: "Secret('**********')"
print(config.api_key.get_secret_value()) # Would return: "sk_test_abcdef"
```

Or, a secret kwarg on String
```python
import param

class Config(param.Parameterized):
api_key = param.String(default="sk_test_abcdef", secret=True)

config = Config()
print(config.api_key) # Would display: "**********"
print(config.get_secret_value("api_key")) # Would return: "sk_test_abcdef"
```