feat: add a new basic type identity

[IronCore864]

A new "basic" type of identity and "metrics" type of access are added for the upcoming metrics feature.

To expose the upcoming metrics feature over HTTP, we need a certain level of authentication to protect the endpoint. We decided to use HTTP basic authentication for this purpose. A new type of "basic" identity needs to be implemented for this to work, and the access level would be metrics. The basic identity looks like this:

identities:
  bob:
    access: metrics
    basic:
      password: hashed-password-with-salt

Where the password is generated by openssl passed -6. The hashed password will be stored in the state, and when the user accesses the metrics endpoint, they need to set the Authorization header accordingly. Pebble daemon will sha512 hash the password and compare it to the identity stored in the state.

For more details, see the spec here.

[benhoyt]

Let's please add a few more details about the feature to the PR description, and link to the spec (internal, I know).