feat(boto3): Add Lambda extension - #8117

### Description

Removes `AwsCallContext.service_id_hyphenized` from
`sentry_sdk/integrations/boto3/_context.py`. It's still computed on
every call (`service_id.hyphenize()`), but on `major/3.0` nothing reads
it.

It had two readers. One was the `aws.service_id` span tag, which #7261
("Drop tags auto-set by the SDK") removed. The other was the span name:
#7800 ("align span names with OTel conventions") switched that to
`ctx.service_id`, so the hyphenized value lost its last reader. This
removes the slot, the attribute and the `hyphenize()` call.

**Evidence**
- On `major/3.0`, `git grep -w service_id_hyphenized` matches only the
three lines in `_context.py` that set it up.
- The open boto3 extension PRs against `major/3.0` (#8117 SQS, #8137
Lambda) don't reference it.

On `master`, `service_id_hyphenized` is still used for the span name and
the tag, so this only applies to `major/3.0`.

#### Issues
n/a

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via **Ivana Kellyer**.
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Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Ivana Kellyer <ivana.kellyer@sentry.io>

### Description

With #8117 (merged), a synchronous boto3 `Invoke` carries `sentry-trace`
and `baggage` in `ClientContext.custom`. This PR adds the receiving
side: the Lambda integration continues the trace from
`context.client_context.custom`. `ClientContext` wins over event
headers, because a forwarded HTTP event can still carry headers from
further upstream.

`sam local` drops `ClientContext`, so the new `ClientContextTrace` test
function hands the event's `client_context` to the runtime. That way the
container tests can cover it.

This is Pablo's code from `pablo/aws-lambda-client-context-trace`. The
one change: `client_context` and `custom` are read with `getattr`, so a
context object without them falls back to event headers instead of
failing. It's the first of three stacked PRs for the stretch Lambda
work: this PR → trigger detection (#8222) → `queue.process` spans
(#8223).

#### Issues
* resolves: PY-3009 (receiving side; #8117 does the injection)
* Part of #8071

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via @dingsdax.
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---------

Co-authored-by: Pablo Deputter <pablo.deputter@sentry.io>
Co-authored-by: Pablo Deputter <71842639+pabloDeputter@users.noreply.github.com>
Co-authored-by: sentry-junior[bot] <264270552+sentry-junior[bot]@users.noreply.github.com>
Co-authored-by: Johannes Daxböck <114897+dingsdax@users.noreply.github.com>