sql: implement write path for ALTER ROLE ... SET

ALTER ROLE ... SET will now persist the default settings in the
system.database_role_settings table. The behavior matches Postgres in
the following ways:
- RESET does not validate the setting name.
- SET validates both the name and the proposed default value.
- The defaults are stored in a string array, in key=val format.

The privilege checking is slightly different: we diverge from Postgres
by prohibiting users from modifying their own defaults unless they
have CREATEROLE. This is analogous to our restriction that prevents
a user from modifying their own password. Also, the root and admin roles
can't have their defaults set.

There is no release note since this does not yet affect any
user-behavior. Everything in this commit is write-only

Release note: None

pkg/bench/rttanalysis/testdata/benchmark_expectations

@@ -31,13 +31,13 @@ exp,benchmark
 24,CreateRole/create_role_with_2_options
 25,CreateRole/create_role_with_3_options
 23,CreateRole/create_role_with_no_options
-20,DropDatabase/drop_database_0_tables
-29,DropDatabase/drop_database_1_table
-38,DropDatabase/drop_database_2_tables
-47,DropDatabase/drop_database_3_tables
-28,DropRole/drop_1_role
-34,DropRole/drop_2_roles
-40,DropRole/drop_3_roles
+22,DropDatabase/drop_database_0_tables
+31,DropDatabase/drop_database_1_table
+40,DropDatabase/drop_database_2_tables
+49,DropDatabase/drop_database_3_tables
+30,DropRole/drop_1_role
+37,DropRole/drop_2_roles
+44,DropRole/drop_3_roles
 19,DropSequence/drop_1_sequence
 27,DropSequence/drop_2_sequences
 35,DropSequence/drop_3_sequences