string(TIMESTAMP): add %f specifier for microseconds

Extends the string(TIMESTAMP) and file(TIMESTAMP) command with a new %f that return the number of microseconds. This convention is offered by python's datetime module.

As suggested by @nilsgladitz, the implementation is done using libuv that provides the methods with provide the required time resolution in a cross-platform manner:

• uv_gettimeofday()
• uv_fs_stat()

Implements: #19335 (closed)