Allow user to specify target directory for backup artifact

User story:

As a user I want to put multiple pbm snapshots in single directory. That will let me to add snapshots to running PITR backup and will simplify CLI opeations. Also that will match current pbm users expectations.

UI/UX:

We need to add new mandatory field `directory` backup creation form.

Acceptance criteria:

• User is able to put multiple snapshots into the single dir 
• PBM can read this dir 
•  

Out of scope:

Suggested implementation:

1. User specifies directory in UI  (what validation should be applied to dir name?)
2. We add directory name to the artifact model
3. We send backup job start request including directory name (I guess we can just reuse `name` parameter for that)
4. pmm agent starts backup job
5. Once job started pmm angent sends to the pmm server list of arficat files as progress message
   1. For mongoDB it will be `2022-12-14T15:42:23Z.pbm.json` and `2022-12-14T15:42:23Z/`
   2. For mySQL right now we are using name as output file name, we can change it to node_name/service_name/etc+timestamp or something else
6. pmm server adds files list to artifact model
7. For restore pmm server adds directory (we can reuse existing name field) and artifact filenames
8. For artifact deletion we remove only files specified in artifact
9. Old pmm agents should still work as it was before. So if artifact files list is emty that mean backup was created on old pmm agent and backup name == file name

How to test:

Details: