Support base64-encoded DER private keys

[sfc-gh-eworoshow]

What is the current behavior?

Only bytes and RSAPrivateKey types are supported for private keys

What is the desired behavior?

bytes, str (base64-encoded), and RSAPrivateKey types are supported for private keys

How would this improve snowflake-connector-python?

This functionality is useful for passing private keys through environment variables and other configuration mechanisms.

References and other background

No response

[sfc-gh-dszmolka]

hey Eric thanks again for the PR :)

[aonamrata]

Can you add some example for using this in the snowflake documentation?