[multitenant_provider] Add option for a ttl when request a tenant token

[esune]

Transferring from bcgov/traction#972 now that the multitenant_provider plugin is hosted here.

Quoting @PatStLouis

I would like to see an optional ttl field in the request body for the /multitenancy/tenant/{tenant_id}/token endpoint.
Some applications might require a token with an expiration date of a desired length set.

[PatStLouis]

I was thinking an optional value which would overwrite the default value when calculating the exp if defined.

It should be picked up when verifying the token expiration.

I will have a look at a PR.