dvc: introduce dvc params diff

[efiop]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here. If the CLI API is changed, I have updated tab completion scripts.

@dmpetrov Volunteered to provide docs for this with nice examples 🙏

• ❌ I will check DeepSource, CodeClimate, and other sanity checks below. (We consider them recommendatory and don't expect everything to be addressed. Please fix things that actually improve code or fix bugs.)

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[efiop]

Issues:

angles:          
    - 2          
    - 3          
    - 100        
    - 3.2        
    - 36.2       
    - 3.2        
df3: [1, 3]      
train:           
    lr: 0.0042   
    layers: 10   
    dropout: 0.21
processing:      
    th: 0.98     
    bow: 7000    

diff for angles will be shown as angles.N where N is the number of the element that changed in the list. Need to treat them as lists. EDIT: fixed

And if you change train.lr to 0.0042, diff will show it as 0.004. These affect metrics diff too. EDIT: fixed.

[Suor]

@efiop when do you expect this to be merged?

[efiop]

@Suor Need to implement support for targets (will do today) and get an approval from @dmpetrov . So maybe later today.

[efiop]

Ok, got aproval from Dmitry. Merging.