add support for endpoints that require jwt_token

Adds jwt_token attribute to GhApi to allow users to call app endpoints that require a jwt token instead of a pat/access_key

closes #85

This should allow you to authenticate to the following endpoints:

apps.get_authenticated(): Get the authenticated app
apps.list_installations(per_page, page, since, outdated): List installations for the authenticated app
apps.get_installation(installation_id): Get an installation for the authenticated app
apps.delete_installation(installation_id): Delete an installation for the authenticated app
apps.create_installation_access_token(installation_id, repositories, repository_ids, permissions): Create an installation access token for an app
apps.suspend_installation(installation_id): Suspend an app installation
apps.unsuspend_installation(installation_id): Unsuspend an app installation
apps.get_org_installation(org): Get an organization installation for the authenticated app
apps.get_repo_installation(owner, repo): Get a repository installation for the authenticated app
apps.get_user_installation(username): Get a user installation for the authenticated app
apps.get_webhook_config_for_app(): Get a webhook configuration for an app
apps.update_webhook_config_for_app(url, content_type, secret, insecure_ssl): Update a webhook configuration for an app
apps.get_subscription_plan_for_account_stubbed(account_id): Get a subscription plan for an account (stubbed)
apps.list_plans_stubbed(per_page, page): List plans (stubbed)
apps.list_accounts_for_plan_stubbed(plan_id, sort, direction, per_page, page): List accounts for a plan (stubbed)

And probably more, these were just the ones I quickly tested.

For example:

jwt = create_jwt(app_id, private_key)

app_api = GhApi(jwt_token=jwt)

print("List Installations")
installations = app_api.apps.list_installations()
print(installations)

print("Get Access Token")
id = installations[0]['id']
print(app_api.apps.create_installation_access_token(id))

List Installations

[- id: 1111
- account: 
  - login: ffalor
  - id: 1
  - node_id: MDQ6VXNlcjM1MTQ0MTQx
  #etc...]

Get Access Token
- token: ghs_324324234234
- expires_at: 2021-08-21T11:40:36Z
- permissions: 
  - administration: read
  - contents: read
  - issues: write
  - metadata: read
- repository_selection: all

Not 100% what all I can update since from my understanding this project is auto generated. I did not see any tests to update. Please let me know what would need to change in order to add the ability to provide a jwt_token.

fastai / ghapi

add support for endpoints that require jwt_token #87