SM-1023: Remove Instant from Client in SDK

[coltonhurst]

Type of change

- [ ] Bug fix
- [ ] New feature development
- [x] Tech debt (refactoring, code cleanup, dependency upgrades, etc)
- [ ] Build/deploy pipeline (DevOps)
- [ ] Other

Objective

Refactor out the Instant type in Client and use unix timestamps. This is to support this PR: #388 where we need to serialize token expiry times.

It also brings benefits to other areas like WASM, where Instant doesn't exist.

Code changes

• crates/bitwarden/src/auth/renew.rs: Update the renew_token function to use timestamps instead of the Instant type
• crates/bitwarden/src/client/client.rs: Update Client to store the unix timestamp for token expiration

Before you submit

• Please add unit tests where it makes sense to do so (encouraged but not required)

[bitwarden-bot]

Checkmarx One – Scan Summary & Details – bd033fff-7cce-42b4-8da3-499fec872153

No New Or Fixed Issues Found

[Hinton]

I think we could refactor the logic around expiration date to. The JWT token contains the expiration date, not sure why we map it back and forth.