Add missing support for the latest SPI status codes.
SPI_result_code_string() was missing support for SPI_OK_TD_REGISTER,
and in v15 and later, it was missing support for SPI_OK_MERGE, as was
pltcl_process_SPI_result().
The last of those would trigger an error if a MERGE was executed from
PL/Tcl. The others seem fairly innocuous, but worth fixing.
Back-patch to all supported branches. Before v15, this is just adding
SPI_OK_TD_REGISTER to SPI_result_code_string(), which is unlikely to
be seen by anyone, but seems worth doing for completeness.
Reviewed by Tom Lane.
Discussion:
https://postgr.es/m/CAEZATCUg8V%2BK%2BGcafOPqymxk84Y_prXgfe64PDoopjLFH6Z0Aw%40mail.gmail.com
https://postgr.es/m/CAEZATCUMe%2B_KedPMM9AxKqm%3DSZogSxjUcrMe%2BsakusZh3BFcQw%40mail.gmail.com
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