tracing: utxocache tracepoints

Summary:

The tracepoints are further documented in docs/tracing.md and the usage is shown via the two newly added example scripts in contrib/tracing/.

Backport of core#22902.
Backport of core#23907.

Notes:

• The amount is passed as string because our type can't be converted to int
• CTxOut and CCoin accessors are used where applicable
• I removed the casts that are both useless and even mixing stding and posix types

Test Plan:
Build with tracing, run bitcoind, then:

sudo ../contrib/tracing/log_utxocache_flush.py ./src/bitcoind
sudo bpftrace ../contrib/tracing/log_utxos.bt

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12715