Add omdb db validate subcommands (#4376)

In order to diagnose if customer-support#57 is caused by _not_ having
the fixes for omicron#3866, add a few commands to omdb to validate the
contents of the database:

    Usage: omdb db validate <COMMAND>

    Commands:
      validate-volume-references  Validate each `volume_references` column in the region snapshots table
      validate-region-snapshots   Find either region snapshots Nexus knows about that the corresponding Crucible agent says were deleted, or
                                      region snapshots that Nexus doesn't know about
      help                        Print this message or the help of the given subcommand(s)

This commit also adds an environment variable OMDB_FETCH_LIMIT, which
overrides the default fetch limit.

dev-tools/omdb/Cargo.toml

@@ -12,6 +12,7 @@ anyhow.workspace = true
 async-bb8-diesel.workspace = true
 chrono.workspace = true
 clap.workspace = true
+crucible-agent-client.workspace = true
 diesel.workspace = true
 dropshot.workspace = true
 futures.workspace = true
@@ -39,10 +40,10 @@ tokio = { workspace = true, features = [ "full" ] }
 uuid.workspace = true
 ipnetwork.workspace = true
 omicron-workspace-hack.workspace = true
+nexus-test-utils.workspace = true
 
 [dev-dependencies]
 expectorate.workspace = true
-nexus-test-utils.workspace = true
 nexus-test-utils-macros.workspace = true
 omicron-nexus.workspace = true
 omicron-test-utils.workspace = true