feat: add DocumentStoreSSHClient

[helen-m-lin]

closes #44

• Added new DocumentStoreSSHClient for users to interface with DocDB through SSH tunneling
• Credentials are pulled from environment on default, or can be configured by provided the secret names
• Can be used in context, e.g.:

with DocumentStoreSSHClient(credentials=DocumentStoreSSHCredentials()) as ds_client:
    count = ds_client.collection.count_documents({})

[helen-m-lin]

TODO: update the readme example to use a projection and count_documents
TODO: rename module and classes to DocumentDB instead of DocumentStore

[helen-m-lin]

TODO: update the readme example to use a projection and count_documents TODO: rename module and classes to DocumentDB instead of DocumentStore

@jtyoung84 I updated with the suggestions