Change: additional arguments/assertions for UC24 and OEM 24.04 via Zapper KVM

Description

This PR addresses a few problems detected while provisioning a device with UC24-subiquity and Desktop 24.04 OEM.

About UC24:

being a non-dangerous grade image, the installed user will be given by ubuntu_sso_email. This PR forwards the ubuntu_sso_email argument to the zapper.provision() API and assert that test_username matches the local part of the SSO email. The DUT pulls TF SSH key from the account, so it's accessible and reservation works fine.

About 24.04 OEM:

user_data_base was intended as a plain string, making it difficult to include in a TF job yaml. Taking inspiration from the MAAS connector I made it a base64 encoded field. Added tests and assertions.
being a common case, instead of asking the user to pass the user_data_base argument, I added a dedicated autoinstall_oem boolean flag. It will be handled on the Zapper side as described in the autoinstall docs. See https://github.com/canonical/zapper/pull/311 for reference on what it actually does on the Zapper side.

Resolves ZAP-873 Part of ZAP-776

Updated README and docs/.

N/A

About UC24: couldn't really test it e2e but, once manually provisioned via Zapper API, the device is accessible as usual by TF with:

~ cat uc24.yaml       
job_queue: 202101-28611
**test_data:
  test_username: ce-certification-qa**
reserve_data:
  ssh_keys:
    - lp:pgentili

About 24.04 OEM: tested on the Zapper side as part of https://github.com/canonical/zapper/pull/311 passing oem=True.