Permit bearer_token field to be JSON

[bilbof]

This enables the bearer_token option for the JsonClient to be a JSON with the structure '{"value": <string>}'.

Trello: https://trello.com/c/6Uaayg1l/545-use-a-single-rotation-lambda-instead-of-duplicating-them-per-secret

Motivation

We would like to store additional data with the secret body, to enable rotation of the bearer token with configuration for the rotation provided in the secret body.

E.g. the secret body may look like this:

{ "api_user": "publisher@alphagov.co.uk", "value": "123", "application": "publishing-api" }

The value field would contain the bearer token string. The remaining fields would enable rotation by a Lambda.

This should not have an impact on the existing platform. I expect secrets stored in govuk-secrets to remain unchanged. This will only impact secrets stored in AWS SecretsManager for the new ECS-hosted platform.

[bilbof]

This isn't necessary, since we can specify json-key in the ARN provided in the valueFrom field: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/specifying-sensitive-data-secrets.html#secrets-envvar

Thanks @sengi!