Make token TTL configurable

Currently `TOKEN_EXPIRY_SECS` is hardcoded to `48 * 3600` [code link](https://github.com/aws/aws-nitro-enclaves-acm/blob/main/src/vtok_common/src/lib.rs#L27). 

Can we make it configurable?

I can make a PR for this.  

I am thinking of adding the token expiry ttl in Token struct used in API schema (https://github.com/aws/aws-nitro-enclaves-acm/blob/main/src/vtok_rpc/src/api.rs#L149)
