Rename "Postgres nodes" in control_plane to endpoints.

We use the term "endpoint" in for compute Postgres nodes in the web UI
and user-facing documentation now. Adjust the nomenclature in the code.

This changes the name of the "neon_local pg" command to "neon_local
endpoint". Also adjust names of classes, variables etc. in the python
tests accordingly.

This also changes the directory structure so that endpoints are now
stored in:

    .neon/endpoints/<endpoint id>

instead of:

    .neon/pgdatadirs/tenants/<tenant_id>/<endpoint (node) name>

The tenant ID is no longer part of the path. That means that you
cannot have two endpoints with the same name/ID in two different
tenants anymore. That's consistent with how we treat endpoints in the
real control plane and proxy: the endpoint ID must be globally unique.

test_runner/performance/test_layer_map.py

@@ -25,8 +25,8 @@ def test_layer_map(neon_env_builder: NeonEnvBuilder, zenbenchmark):
     )
 
     env.neon_cli.create_timeline("test_layer_map", tenant_id=tenant)
-    pg = env.postgres.create_start("test_layer_map", tenant_id=tenant)
-    cur = pg.connect().cursor()
+    endpoint = env.endpoints.create_start("test_layer_map", tenant_id=tenant)
+    cur = endpoint.connect().cursor()
     cur.execute("create table t(x integer)")
     for i in range(n_iters):
         cur.execute(f"insert into t values (generate_series(1,{n_records}))")