Add support for retrieving JSON documents with VAULT[] syntax

[elrzn]

Hi!

Currently, trying to retrieve a secret stored as a JSON document instead of a string will raise the following error:

Vault-boxed value has an invalid format: selector's element "secret" should point to a string.

Since Vault has native support for storing secrets as JSON documents, it would be a great QoL feature to have. This makes sense for example when using GCP service accounts, without having to encode them, which would make version controlling of secrets more difficult.

I personally wouldn't mind something like VAULT_JSON[...] if it helps to make the implementation easier.

[edigaryev]

Reopening since the changes are not deployed yet.

[fkorotkov]

It got rolled out. 👌