Context manager for contiguous computations

## Motivation

For example, groups (like us!) run benchmarks and want to group sets of computations together into one cohesive job. We also want to record information about that computation that we know, for example the name of the benchmark, the hardware on which it was run, etc..

## Example

```python
with client.computation(tags={...}):
df = df.persist()

out1 = df.x.sum().compute()
out2 = df.y.sum().compute()
```

This name `computation` is bad. We should find a better one.

Question: should this be allowable without a context manager? @crusaderky is mildly positive on this.

What should happen here?: these should not nest. That should raise an errors

```python
with client.computation():
with client.computation():
```

@crusaderky @hendrikmakait @j-bennet