Align and flesh out trace propagation API

[limbonaut]

The trace propagation spec's Public API section wants three methods: continue_trace, get_trace_data, and start_new_trace (naming can vary). Native already has most of the underlying behavior, but only the trace reset API is directly reachable from public.

continue_trace is not provided without starting a transaction.
sentry_transaction_context_update_from_header() already parses sentry-trace and baggage and checks sentry-org_id for strict continuation, but it writes into a transaction context and is only applied by the transaction-start path. The spec requires a public continuation API that updates trace propagation state directly and MUST NOT create a segment span (aka transaction or topmost span).

The method MUST NOT create a new segment span on its own.

get_trace_data is not provided without an active span.
sentry_span_iter_headers() and sentry_transaction_iter_headers() both require a live span or transaction. Similar gap as in the previous issue.

start_new_trace exists as sentry_regenerate_trace().
Already correct. Consider renaming for clarity.

Without spans there is no way to propagate a trace at all. The spec says an SDK with no traces_sample_rate must still continue incoming traces and send sentry-trace and baggage (default propagation), and deferred sampling describes what to send when no span is available.

Proposal

I marked API that we need in the Godot SDK with // [G].

// New API for continuing a trace from headers.
SENTRY_API void sentry_continue_trace(
    const char *sentry_trace, const char *baggage); // [G]
SENTRY_API void sentry_continue_trace_n(
    const char *sentry_trace, size_t sentry_trace_len,
    const char *baggage, size_t baggage_len);
// Same, for a given scope, so concurrent units of work can hold distinct traces.
SENTRY_API void sentry_scope_continue_trace(sentry_scope_t *scope,
    const char *sentry_trace, const char *baggage); // [G]
SENTRY_API void sentry_scope_continue_trace_n(sentry_scope_t *scope,
    const char *sentry_trace, size_t sentry_trace_len,
    const char *baggage, size_t baggage_len);

// The missing scope twin of sentry_set_trace.
SENTRY_API void sentry_scope_set_trace(sentry_scope_t *scope,
    const char *trace_id, const char *parent_span_id); // [G]
SENTRY_API void sentry_scope_set_trace_n(sentry_scope_t *scope,
    const char *trace_id, size_t trace_id_len,
    const char *parent_span_id, size_t parent_span_id_len);

// Reads the global scope: its bound span if it has one, otherwise 
// the propagation context.
SENTRY_API void sentry_iter_trace_propagation_headers(
    sentry_iter_headers_function_t callback, void *userdata);
// Same, for a given scope, falling back to the global scope for 
// what it does not set.
SENTRY_API void sentry_scope_iter_trace_propagation_headers(
    const sentry_scope_t *scope,
    sentry_iter_headers_function_t callback, void *userdata); // [G]

// Rename for clarity.
SENTRY_API void sentry_start_new_trace(void);
SENTRY_DEPRECATED("Use `sentry_start_new_trace` instead")
SENTRY_API void sentry_regenerate_trace(void);

[linear-code]

NATIVE-217