fix(work pools): fix work pool base job template "inconsistent values"

[mitchnielsen]

Summary

• Addresses the "inconsistent values" error the provider would sometimes return when working with work pools that specify a base job template (context: https://github.com/PrefectHQ/terraform-provider-prefect/issues/244#issuecomment-2305902561)
• Adds base_job_template to the Work Pool tests
• Conforms to the newer, simpler method of unmarshaling the base job template that we use in our Block data logic

More information on each change is available in the relevant commit message.

Closes https://github.com/PrefectHQ/terraform-provider-prefect/issues/244