UX: adding an executor requires grabbing the default list

Nobody has claimed this yet.

## 🚀 Feature

### Motivation

Adding an executor requires obtaining the list of default executors and appending them, which is not very elegant:

```py
# Default executors
model = thunder.jit(model, executors=None) # None means default

# Adding Transformer Engine
te = thunder.extend.get_executor("transformer_engine")
executors = [te] + list(get_default_executors())
model = thunder.jit(model, executors=executors)

```

### Pitch

Allow providing only additional executors.

```py
# Proposition
te = thunder.extend.get_executor("transformer_engine")
model = thunder.jit(model, additional_executors=[te])

```

**Open questions**: how to retain full control on executors, i.e. how to allow user to modify the list of default executors? This is possible now, but won't be possible with the proposed solution.

### Alternatives

### Additional context

Discussed with @tfogal et al who requested to create this issue.

Start with the thunder.jit executor argument and the thunder.extend.get_executor and get_default_executors entry points shown in the issue. Define how additional executors are combined with defaults while preserving a way for users to modify the full executor list; done means the proposed usage works without removing that control.

Written by the indexing model from the issue text.