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

doc/src/sgml/libpq.sgml

@@ -10318,21 +10318,21 @@ typedef struct _PGpromptOAuthDevice
          of <symbol>PGoauthBearerRequest</symbol>, which should be filled in
          by the implementation:
 <synopsis>
-typedef struct _PGoauthBearerRequest
+typedef struct PGoauthBearerRequest
 {
     /* Hook inputs (constant across all calls) */
-    const char *const openid_configuration; /* OIDC discovery URL */
-    const char *const scope;                /* required scope(s), or NULL */
+    const char *openid_configuration; /* OIDC discovery URL */
+    const char *scope;                /* required scope(s), or NULL */
 
     /* Hook outputs */
 
     /* Callback implementing a custom asynchronous OAuth flow. */
     PostgresPollingStatusType (*async) (PGconn *conn,
-                                        struct _PGoauthBearerRequest *request,
+                                        struct PGoauthBearerRequest *request,
                                         SOCKTYPE *altsock);
 
     /* Callback to clean up custom allocations. */
-    void        (*cleanup) (PGconn *conn, struct _PGoauthBearerRequest *request);
+    void        (*cleanup) (PGconn *conn, struct PGoauthBearerRequest *request);
 
     char       *token;   /* acquired Bearer token */
     void       *user;    /* hook-defined allocated data */

src/interfaces/libpq/libpq-fe.h

@@ -745,11 +745,11 @@ typedef struct _PGpromptOAuthDevice
 #define SOCKTYPE int
 #endif
 
-typedef struct _PGoauthBearerRequest
+typedef struct PGoauthBearerRequest
 {
 	/* Hook inputs (constant across all calls) */
-	const char *const openid_configuration; /* OIDC discovery URI */
-	const char *const scope;	/* required scope(s), or NULL */
+	const char *openid_configuration;	/* OIDC discovery URI */
+	const char *scope;			/* required scope(s), or NULL */
 
 	/* Hook outputs */
 
@@ -770,7 +770,7 @@ typedef struct _PGoauthBearerRequest
 	 * request->token must be set by the hook.
 	 */
 	PostgresPollingStatusType (*async) (PGconn *conn,
-										struct _PGoauthBearerRequest *request,
+										struct PGoauthBearerRequest *request,
 										SOCKTYPE * altsock);
 
 	/*
@@ -780,7 +780,7 @@ typedef struct _PGoauthBearerRequest
 	 * This is technically optional, but highly recommended, because there is
 	 * no other indication as to when it is safe to free the token.
 	 */
-	void		(*cleanup) (PGconn *conn, struct _PGoauthBearerRequest *request);
+	void		(*cleanup) (PGconn *conn, struct PGoauthBearerRequest *request);
 
 	/*
 	 * The hook should set this to the Bearer token contents for the