Generalize system variables

[apstndb]

It is a random idea about general system variable concept.

It may be overgeneralize for current status, but I think it is worth to remember when feature enhancement become hard.

This issue is forked from this comment: #183 (comment)

Motivation

Although spanner-cli has evolved alongside Cloud Spanner as a product, it can be difficult to extend its functionality consistently.

• Some features are implemented as spanner-cli statements.
• Some features are implemented as go-flags options.
• Some of them can't be changed in a spanner-cli session.
• The current value of some of them can't be displayed.
  • No prompt variables and no show commands.

These could be barriers to future feature expansion.

I assume that introducing general system variable may solve some problem.

• Generalize some statements into SET, and SHOW VARIABLE.
  • spanner-cli can borrow JDBC session management commands (GoogleSQL) of the Cloud Spanner JDBC driver.
    • It may help users who use both of spanner-cli and JDBC drivers.
    • Additionally, some of features in the JDBC drivers are not yet implemented in spanner-cli, it may help future discussion of enhancement.
• Generalize command line options.
  • We can borrow psql style variable flag.
    • -v assignment
    • --set=assignment
    • --variable=assignment
• SET statement can be used in spanner-cli session.
  • spanner-cli can reject it when modifying is non-sense.
• System variables can be displayed as generalized prompt variable like \v{VARIABLE_NAME}.
  • SHOW VARIABLE statement can display system variable value as on-demand.