## Description 

This allows even single-owner transactions to have access to a concept
of time that ticks with each epoch change.

## Test Plan 

```
cargo nextest run
```

+ New test for the test scenario changes:

```
cargo nextest run -- 'run_framework_move_unit_tests'
```

### Type of Change (Check all that apply)

- [x] user-visible impact
- [ ] breaking change for a client SDKs
- [x] breaking change for FNs (FN binary must upgrade)
- [x] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

Expose the start timestamp of the epoch that the transaction is running
in via the `TxContext`, using `sui::tx_context::epoch_timestamp(ctx:
&TxContext): u64`. This concept of time is available even for
single-owner transactions.