fix(upload): error on upload with the same filename

[tessus]

Description

Currently it is possible to upload files with the same name. This is undefined behavior. e.g. which file should be returned when accessing it?

To solve this problem this change introduces a check whether a file (that hasn't expired) alredy exists with the same name. If so, an error will be returned.

Motivation and Context

see https://github.com/orhun/rustypaste/discussions/241

I am opening this PR now, because I want to check, whether the test that fails in #246 also fails with this one.

Depending on which PR is merged first, a change will be required to add an .await to the glob_match_file call.

How Has This Been Tested?

• manually
• cargo tests (new tests)
• fixtures (new tests)

Changelog Entry

### Changed

- Return an error when a file with the same name already exists on the server

Types of Changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (no code change)
• [ ] Refactor (refactoring production code)
• [ ] Other

Checklist:

• [x] My code follows the code style of this project.
• [ ] I have updated the documentation accordingly.
• [x] I have formatted the code with rustfmt.
• [x] I checked the lints with clippy.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 70.21%. Comparing base (fa5105d) to head (1266766).

Files	Patch %	Lines
src/paste.rs	83.33%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #258 +/- ## ========================================== + Coverage 70.09% 70.21% +0.12% ========================================== Files 11 11 Lines 632 638 +6 ========================================== + Hits 443 448 +5 - Misses 189 190 +1 ``` | [Flag](https://app.codecov.io/gh/orhun/rustypaste/pull/258/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z) | Coverage Δ | | |---|---|---| | [unit-tests](https://app.codecov.io/gh/orhun/rustypaste/pull/258/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z) | `70.21% <83.33%> (+0.12%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z#carryforward-flags-in-the-pull-request-comment) to find out more.

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