Migration CLI: Add migrate-get command and --dry-run/--show-sql options

[brandur]

This one's in pursuit of resolving #209, which I've decided to tackle
now because we're going to have to cut a CLI release across River
package versions for #258 anyway, so this'll reuse some work.

A new river migrate-get command becomes available, whose only job is
to dump SQL from River migrations so that it can easily be plugged into
other migration frameworks. Its use looks like:

river migrate-get --version 3 --down > version3.down.sql
river migrate-get --version 3 --up > version3.up.sql

It can also take multiple versions:

river migrate-get --version 3,2,1 --down > river.down.sql
river migrate-get --version 1,2.3 --up > river.up.sql

It can also dump all migrations, which will be useful in cases where
users want to avoid River's internal migration framework completely, and
use their own:

river migrate-get --all --exclude-version 1 --up > river_all.up.sql
river migrate-get --all --exclude-version 1 --down > river_all.down.sql

Along with that, migrate-down and migrate-up get a few new useful
options:

• --dry-run: Prints information on migrations that would be run, but
  doesn't modify the database in any way.

• --show-sql: Prints SQL for each migration step that was applied.

This gives users an easy way to, after a River upgrade, run the CLI to
see what commands would be run were they to migrate, but without
actually performing the migration, likely a step that most production
users would perform to be cautious:

river migrate-up --dry-run --show-sql

I've also done a little cleanup around the River CLI's main.go. The
--verbose and --debug commands added in #258 are now promoted to
persistent flag configuration so they're available for all commands, and
we now have one standardized way of initializing an appropriate logger.

Fixes #209.

[brandur]

It's a little odd that this command takes --down and --up options, when the migration commands are migrate-down and migrate-up, but I tried to consider all the alternatives, and I think this is the least bad. Having two full commands for migrate-get-down and migrate-get-up feels too heavy handed and unwieldy.

[brandur]

Ugh, I wish I could write more tests for the CLI, but I'm getting hard blocked by the fact that none of our internal test utilities are available to this module. Something to look into separately, but punting on it for now.

[brandur]

thx!