pw_multisink: Lock multisink operations

Guards all multisink transactions with a lock. The new configuration
option PW_MULTISINK_LOCK_INTERRUPT_SAFE allows the project to select
the type of lock used to guard transactions. By default, it is enabled
and makes use of an interrupt spin-lock. If disabled, a mutex is used
instead.

Change-Id: I71ab2729d130c524da27e0d06beb0c3fdf73d145
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/45720
Commit-Queue: Prashanth Swaminathan <prashanthsw@google.com>
Pigweed-Auto-Submit: Prashanth Swaminathan <prashanthsw@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>