Implement dotfiles

[kyujin-cho]

This PR resolves issue lablup/backend.ai#98.

What has changed?

• Added dotfiles column to keypairs table
• New /dotfile endpoint for basic CRUD operation of dotfiles
  • GET /dotfile: Lists all available dotfiles per user
  • GET /dotfile?path=<SOMEPATH>: Returns information of dotfile with given path
  • POST /dotfile: Creates new dotfile
  • PATCH /dotfile: Updates dotfile with given patgh
  • `DELETE /dotfile?path=: Deletes dotfile from user with given path
• Manager now passes dotfiles list inside internal_data column to agent when creating kernel

[codecov]

Codecov Report

Merging #220 into master will increase coverage by 0.1%.
The diff coverage is 68%.

@@            Coverage Diff            @@
##           master     #220     +/-   ##
=========================================
+ Coverage   45.87%   45.97%   +0.1%     
=========================================
  Files          36       36             
  Lines        5783     5805     +22     
=========================================
+ Hits         2653     2669     +16     
- Misses       3130     3136      +6

Impacted Files	Coverage Δ
src/ai/backend/gateway/server.py	52.56% <ø> (ø)	⬆️
src/ai/backend/manager/registry.py	13.48% <0%> (-0.05%)	⬇️
src/ai/backend/gateway/exceptions.py	87.07% <100%> (+0.68%)	⬆️
src/ai/backend/manager/models/keypair.py	45.36% <66.66%> (+1.09%)	⬆️

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[achimnol]

I'd like to put the dotfiles APIs under "user-config" API set.
So, please rename gateway/dotfile.py to gateway/userconfig.py and use additional prefix levels /user-config/dotfiles instead of /dotfile.