…lls (elastic#242727)

## Summary

Closes elastic#242716

This PR updates the Trace Errors API calls for APM, and splits the
`getApmTraceError` call from the Unified Trace APIs to have a new
`getUnifiedApmTraceError` call, which allows to unify the return
response for the Unified Trace APIs, while maintaining backwards
compatibility with the old APM APIs. Both Error methods now make use of
`accessKnownApmEventFields`, with `getApmTraceError` better handling the
unflattening and response construction, and `getUnifiedApmTraceError`
querying for less fields and avoiding the use of unflattening entirely.
This now has the Unified Trace APIs no longer using any unflattening in
their call stack, and standardises the error interface between APM and
OTEL errors.

## How to test

- Go to Discover on an Observability space, select a traces index (APM
and/or OTEL) in either Classic or ES|QL.
- Select a trace with either an `event.outcome` as `failure` or
`status.code` as `Error`. On spans/transactions with an error attached
to it, it should show on both the focused and full trace waterfall
without any issue.
- Going through to the APM page via the `trace.id` link, the APM trace
waterfall should also show the same errors with no issue.

(cherry picked from commit 505fe7b)

# Conflicts:
#	x-pack/solutions/observability/plugins/apm/server/routes/traces/get_trace_items.ts
#	x-pack/solutions/observability/plugins/apm/server/routes/traces/get_unified_trace_errors.ts
#	x-pack/solutions/observability/plugins/apm/server/routes/traces/get_unified_trace_items.test.ts
#	x-pack/solutions/observability/plugins/apm/server/routes/traces/get_unified_trace_items.ts
#	x-pack/solutions/observability/plugins/apm/server/routes/traces/normalize_errors.test.ts
#	x-pack/solutions/observability/plugins/apm/server/routes/traces/normalize_errors.ts