Check max jwt exp

[elliesterner]

Checks if the max_exp_jwt of the GCP role is less than the duration of the tests, an error will be returned immediately.

Waiting on merging of these 2 PRs:

• Alter BuildTargets and Setup function signatures to accept a struct with more info #138
• Add gcp auth target #131

2023-07-20T08:30:38.282-0500 [INFO]  vault-benchmark: setting up targets
2023-07-20T08:30:38.286-0500 [ERROR] vault-benchmark: target setup failed: GCP role max_jwt_exp (2s) cannot be shorter than benchmark test duration(20m0s)

[elliesterner]

closing in favor of: #149