string(TIMESTAMP): add %f specifier for microseconds

The %f specified extends the string(TIMESTAMP) and file(TIMESTAMP)
commands to output the timestamp with a microsecond resolution.
This convention is offered by python's datetime module.
Before, the precision was limited to seconds.

The implementation is done by extending existing cmTimestamp methods
with a `microseconds` parameter. This parameter is optional in order to
be backwards compatible. The timestamps are now received in a
cross-platform manner using libuv, since the standard C functions like
time() don't allow for sub-second precision.

This requires libuv 1.28 or higher.  We already require higher than
that on Windows, so update the required version for other platforms.

Implements: #19335

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