feat: lock file while read, write and refresh token

[natthan-pigoux]

closes #133

Tests can be performed on read/write_credentials, and the client (probably simpler if we do it through get_token directly). Test cases:

• (read/write_credentials and get_token) multiple processes attempting to read or write to the credentials file simultaneously.
• (read/write_credentials) credentials file is missing, empty, invalid.
• (get_token) token exists and is valid, credentials file is missing, empty, invalid, token needs to be refreshed, refresh token valid/invalid.

[aldbr]

@natthan-pigoux

• I fixed the mypy issues
• I split the test in 2 different files to avoid importing diracx-client within diracx-core
• I simplified a few parts in the tests (by using the tmp_path fixture for instance)
• The logic to test file locking looked robust in terms of evolutivity but sounds complex to debug and flaky (e.g. use of time.sleep). I decided to mock flock instead, it's less robust (we would need change the tests if we use something else), but it looks like more deterministic, quicker, and easier to understand. If you are against that change, we can discuss.