Expose config options as instance scoped sysvars

[morgo]

Enhancement

This is a sibling to #33769

It is suggested that the following config options be converted to the corresponding instance scoped sysvars. This is "Stage 2" of the instance scope proposal.

However, since this proposal was authored it was decided that we will "default to global", and thus many variables will instead be converted to global scoped sysvars and not instance scoped. Those on this list have a strong use case for instance scope (refer to the resources of the local machine).

Config Name	Sysvar Name	Notes	PR
token-limit	tidb_connection_concurrency_limit	Discussed with PM: This should be instance scope since it refers to the resources of an individual server. Forked to #34583	#34662
tmp-storage-path	tidb_tmp_storage_path	Should be forked to instance scope (refers to local path). Needs to be available as a sysvar to be consistent with tidb_mem_oom_action and tidb_mem_usage_alarm_ratio
tmp-storage-quota	tidb_tmp_storage_quota	Should be forked to instance scope (refers to local path). Needs to be available as a sysvar to be consistent with tidb_mem_oom_action and tidb_mem_usage_alarm_ratio
run-ddl	tidb_enable_ddl ?	It makes a lot of sense that you might want to disable DDL from running on a node without restarting it. It should be instance scoped. The name needs discussion. All other "tidb_enable_XXX" are global scoped.
max-server-connections	max_connections	There is a mysql variable which handles this use case fine. We should use it.