refactor: rename data_dir to instance_dir, admin.sock to admin.socket

Picodata follow-up

• Config File Changes:

data_dir renamed to instance_dir

• Source Code Changes:

data_dir renamed to instance_dir

admin.sock renamed to admin.socket

• Environment Variable Changes:

PICODATA_DATA_DIR renamed to PICODATA_INSTANCE_DIR

PICODATA_ADMIN_SOCK renamed to PICODATA_ADMIN_SOCKET

• Follow-up for picodata!1529 (merged)
• See also picodata#1119 (closed)
• Doc pages to change: everywhere - https://docs.picodata.io/picodata/devel/reference/config/#instance_data_dir

https://docs.picodata.io/picodata/devel/tutorial/connecting/#admin_console_connect