Add repos[].if_exists run configuration option

[un-def]

One of:

• error - the new default since 0.20, fail with an error
• skip - the non-configurable pre-0.20 action

Part-of: #3124

[un-def]

type: task
name: task
repos:
  - url: https://github.com/dstackai/dstack
    path: .
    #if_exists: error  # the default action
    #if_exists: skip
commands:
  - ls -1a

0.20.0 (dev) client with if_exists: error or without the option, CLI output:

Exited (none)
Failed to set up the repo (setup repo: repo dir is not empty: /)
Check dstack logs -d task for more details.

0.19.40 client, dstack logs -d task output

level=info msg=Run job log_level=debug
level=debug msg=repo_exists_action is not set, using legacy 'skip' action
level=info msg=Skipping repo checkout: repo dir is not empty path=/
level=info msg=Job state changed new=running

0.20.0 (dev) client with if_exists: skip, dstack logs -d task output

level=info msg=Run job log_level=debug
level=info msg=Skipping repo checkout: repo dir is not empty path=/
level=info msg=Job state changed new=running

[r4victor]

There is a function for that

dstack/src/dstack/_internal/utils/common.py

Line 317 in f26c6f0

def list_enum_values_for_annotation(enum_class: type[enum.Enum]) -> str: