worker_spi: Expand set of options to start workers

A couple of new options are added to this module to provide more control
on the ways bgworkers are started:
- A new GUC called worker_spi.role to control which role to use by
default when starting a worker.
- worker_spi_launch() gains three arguments: a role OID, a database OID
and flags (currently only BGWORKER_BYPASS_ALLOWCONN).  By default, the
role OID and the database OID are InvalidOid, in which case the worker
would use the related GUCs.

Workers loaded by shared_preload_libraries use the default values
provided by the GUCs, with flags at 0.  The options are given to the
main bgworker routine through bgw_extra.  A test case is tweaked to
start two dynamic workers with databases and roles defined by the caller
of worker_spi_launch().

These additions will have the advantage of expanding the tests for
bgworkers, for at least two cases:
- BGWORKER_BYPASS_ALLOWCONN has no coverage in the core tree.
- A new bgworker flag is under discussion, and this eases the
integration of new tests.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/bcc36259-7850-4882-97ef-d6b905d2fc51@gmail.com

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