Add `all_databases` option to databases. (gobackup#348)

### Summary
This PR extends the `allDatabases` feature (introduced for MySQL in gobackup#347
and MSSQL in gobackup#340) to support all remaining database types that can
benefit from full-database dumps:
**PostgreSQL** - Uses `pg_dumpall` to dump all databases when enabled
**MariaDB** - Omits the `--databases` flag to enable all-database backup
via mariadb-backup
**MongoDB** - Omits the `--db` flag to dump all databases with
`mongodump`
**InfluxDB2** - Omits `bucket` filters to backup all buckets

### Changes
Added `allDatabases` boolean config option to PostgreSQL, MariaDB,
MongoDB, and InfluxDB2
Made the `database` (or `bucket`) config field optional when
`allDatabases: true`
Added validation to require database/bucket config when `allDatabases:
false`
Added comprehensive unit tests for all affected databases

### Example Configuration

```
databases:
  my_postgres:
    type: postgresql
    host: localhost
    username: postgres
    allDatabases: true
```

### Related Issues
Extends gobackup#347 (MySQL allDatabases support)
Related to gobackup#340 (MSSQL allDatabases support)
Addresses gobackup#327 and gobackup#193

I would be so happy to review this changes and merge it with the
existing modifications. @huacnlee @1k-off @aminmr

Add support filename format for compressing (gobackup#334)

add parameter to set filename format like
```
    compress_with:
      filename_format: "db_xxx-2006.01.01.15.00.00"
```

docs: Fix typos.

Fix to ensure `compress_with.type` to default `tar`. (gobackup#313)

Add `args` option to compress. (gobackup#284)

Close gobackup#279

Remove `redis-cli` SAVE result check, the `OK` may not exist in some …

…version. (gobackup#309)

Fixes gobackup#308

Update close-slate-issues.yml

Add handling compression argument for postgres (gobackup#280)

In relation to gobackup#208, for postgres database `pg_dump` [supports
`--compress`
argument](https://www.postgresql.org/docs/current/app-pgdump.html). This
PR adds handling of `compress` from config, along with setting the
correct file name. It saves disk space.

The allowed values for `compress` are: `gzip`, `lz4` and `std`. The
compression level can be set as: `gzip:3`.

notifier: Fix GoogleChat read config not enable. (gobackup#277)

Sorry, I forgot to submit the changes to notifier/base.go to add Google
Chat.

Here's an example Google Chat notifier configuration.

    notifiers:
      googlechat:
        type: googlechat
url: https://chat.googleapis.com/v1/spaces/XXXXX/messages?key=XXXXXX
        method: POST

Add GoogleChat Notifier (gobackup#275)

- Add GoogleChat Notifier Support
- Fix to support empty password for mail notifier.