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

… options

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

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.

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - The River CLI now supports `river bench` to benchmark River's job throughput against a database. [PR #254](https://github.com/riverqueue/river/pull/254).
+- The River CLI now has a `river migrate-get` command to dump SQL for River migrations for use in alternative migration frameworks. Use it like `river migrate-get --up --version 3 > version3.up.sql`. [PR #273](https://github.com/riverqueue/river/pull/273).
+- The River CLI's `migrate-down` and `migrate-up` options get two new options for `--dry-run` and `--show-sql`. They can be combined to easily run a preflight check on a River upgrade to see which migration commands would be run on a database, but without actually running them. [PR #273](https://github.com/riverqueue/river/pull/273).
 - The River client gets a new `Client.SubscribeConfig` function that lets a subscriber specify the maximum size of their subscription channel. [PR #258](https://github.com/riverqueue/river/pull/258).
 
 ### Changed