worker_spi: Use term "dynamic" for bgworkers launched with worker_spi_launch()

worker_spi: Use term "dynamic" for bgworkers launched with worker_spi_launch()

This gives a way to make a difference between workers registered when
the library is loaded with shared_preload_libraries and when these are
launched dynamically, in ps output or pg_stat_activity.

Extracted from a larger patch by the same author.

Author: Bharath Rupireddy
Reviewed-by: Masahiro Ikeda
Discussion: https://postgr.es/m/CALj2ACWR9ncAiDF73unqdJF1dmsA2R0efGXX2624X+YVxcAVWg@mail.gmail.com