oauth: Fix incorrect const markers in struct
Two members in PGoauthBearerRequest were incorrectly marked as const.
While in there, align the name of the struct with the typedef as per
project style.
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/912516.
1740329361@sss.pgh.pa.us