Enforce `storage` and `transfer_tool` uppercase

[ghiggi]

Prework

• [x] I understand and agree to this repository's code of conduct.
• [x] I understand and agree to this repository's contributing guidelines.
• [x] I have already submitted an issue or discussion thread to discuss my idea with the maintainers.

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Documentation
• [ ] Tutorial
• [ ] Code style update
• [x] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and communicate accordingly:

The PR fulfills these requirements:

• [x] It's submitted to the branch named as follow:
  • Fix a bug: bugfix-<some_key>-<word>
  • Improve the doc: doc-<some_key>-<word>
  • Improve a tutorial tutorial-<some_key>-<word>
  • Add a new feature: feature-<some_key>-<word>
  • Refactor some code: refactor-<some_key>-<word>
  • Optimize some code: optimize-<some_key>-<word>
• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [x] Don't forget to link PR to issue if you are solving one.
• [x] All tests are passing.
• [ ] New/updated tests are included

Summary

This PR enforce the storage and transfer_tool acronyms to be upper case in the internal codebase.

• Valid values for storage are: PPS, GES_DISC, LOCAL.
• Valid values for transfer_tool: CURL, WGET

Additionally, the availability of the transfer tools (CURL and WGET) is now tested only once at import time !

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.54545% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 87.66%. Comparing base (0b87161) to head (9025591).

Files	Patch %	Lines
gpm/io/download.py	33.33%	2 Missing :warning:
gpm/io/checks.py	95.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #52 +/- ## ======================================= Coverage 87.66% 87.66% ======================================= Files 111 111 Lines 10520 10534 +14 ======================================= + Hits 9222 9235 +13 - Misses 1298 1299 +1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[coveralls]

coverage: 87.669% (+0.007%) from 87.662% when pulling 18545cd7ecd70e2afbdae7902201111fc006dc00 on refactor-storage-acronym into 0b8716139dc5757de214901273615f79bcf66a55 on main.