Add cosign configuration file

[haydentherapper]

Description

For flag values that remain the same between all invocations of cosign, it'd be useful to have an on-disk config file to persist those values.

Writing the config to disk could be a part of cosign initialize. Additionally, cosign can look in a default location (~/.sigstore/config.yaml) for the config, and this could be customizable via flag.

Persisting state will also be necessary for some BYO TUF features, for example avoiding falling back to the embedded/hosted TUF root when cosign was initialized with a custom root.

cc @asraa