Update private key documentation for dbt-snowflake

[colin-rogers-dbt]

Contributions

• I am a PM or subject matter expert at dbt who is responsible for this feature.

Where does this content belong?

See: second paragraph of Snowflake docs

We just need to add that we support PEM in addition to DER private key formats for dbt versions >= 1.6

Link to source material

Current:
Starting from dbt v1.5.0, you have the option to use a private_key string instead of a private_key_path. The private_key string should be in Base64-encoded DER format, representing the key bytes. Refer to Snowflake documentation for more info on how they generate the key.

Proposed:
Starting from dbt v1.5.0, you have the option to use a private_key string instead of a private_key_path. The private_key string should be in Base64-encoded DER or PEM format, representing the key bytes. Refer to Snowflake documentation for more info on how they generate the key.

[colin-rogers-dbt]

Closing in favor of #3786