Add egress schema to `polykey secrets env`

[aryanjassal]

Specification

Use ajv to create a schema validator for egress of secrets. It should follow a env.schema.json as is drafted in zeta house. Refer to the parent issue for more information

Additional context

• Parent issue: Vault and File Schema - Ingress and Egress Schemas Polykey#222

Tasks

1. Use ajv to read and validate schema
2. Modify env behaviour based on provided schema via a flag
3. Throw error and fail-fast when encountering any error
4. The secrets should not be bulk-fetched and should be streamed per secret as is dictated by the schema

[linear]

ENG-638