Remove secret storage from connector

[dnephin]

Summary

This PR removes infrahq/secrets from the connector. The connector only ever used secrets from env, file, and plaintext, all of which we can read directly with the need of an env:, file:, or plaintext: prefix on the value.

Important: this is technically a breaking change since we no longer support the env: and file: prefixes on access key values. I doubt anyone is using these because it's a special case in the helm chart, and I don't think we ever suggested anyone use it this way.

Also:

• always use a kubernetes secret for the connector access key
• adds support for reading the access key from INFRA_ACCESS_KEY env var in the connector. This matches the CLI. Unlikely anyone will use this yet though, since everyone is using the helm chart, not running the connector directly.
• adds support for reading the connector proxy CA and key from literals, in addition to files. This will be useful at least for writing integration tests of the connector, but may also be useful to others deploying the connector in different envrionments.

TODO:

• [ ] test out the helm chart manually