Add list type for bound claims of vault_jwt_auth_backend_role

[anhdat]

Fix #408

Bound Claims of Vault JWT Auth Backend Role supports both string and list of strings:
https://www.vaultproject.io/api/auth/jwt/index.html#bound_claims
Implementation in vault-plugin-auth-jwt: https://github.com/hashicorp/vault-plugin-auth-jwt/blob/363f5913f469fff63ab979d498ea07a57a5d2639/claims.go#L92

This PR allows user to pass a comma-separated string list, while still backward compatible with single string. For example:

resource "vault_jwt_auth_backend_role" "ops-web" {
  ...

  bound_claims = {
    department = "Engineering"
    division   = "Europe"
    email      = "[email protected],[email protected]"
  }

...
}

[tyrannosaurus-becks]

Thank you!