Add headers for remote spec

[Nexushunter]

With the next gen version of the generator now consumable it would be nice to provide the ability to add custom bearer tokens to the fetch spec requests.

This adds the follow:

• A new base InputSpec type that represents local file access
• A RemoteSpec type that represents a hosted spec file.
  • This is currently only supporting a bearer token and doesn't pull from the environment
  • This could be refined to look for something link OAS_BEARER or something instead of hard coding. I'm not sure yet.
  • An AWS specific AwsRemoteSpec that allows for access to specs in authenticated buckets
  • Localstack for testing aws support

Attempts to solve: https://github.com/gibahjoe/openapi-generator-dart/issues/80

[gibahjoe]

@Nexushunter , seems there is an issue with the workflow configuration. Please take a look so all checks can run

[Nexushunter]

@gibahjoe I'll put up a fix in an hour. Looks like I messed up the merge from master sorry

[Nexushunter]

@gibahjoe It should be fixed now

[codecov[bot]]

Codecov Report

Patch coverage: 73.68% and project coverage change: -1.67% :warning:

Comparison is base (229815e) 81.75% compared to head (677a318) 80.08%. Report is 27 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #98 +/- ## ========================================== - Coverage 81.75% 80.08% -1.67% ========================================== Files 9 9 Lines 570 668 +98 ========================================== + Hits 466 535 +69 - Misses 104 133 +29 ``` | [Files Changed](https://app.codecov.io/gh/gibahjoe/openapi-generator-dart/pull/98?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [...ns/lib/src/openapi\_generator\_annotations\_base.dart](https://app.codecov.io/gh/gibahjoe/openapi-generator-dart/pull/98?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-b3BlbmFwaS1nZW5lcmF0b3ItYW5ub3RhdGlvbnMvbGliL3NyYy9vcGVuYXBpX2dlbmVyYXRvcl9hbm5vdGF0aW9uc19iYXNlLmRhcnQ=) | `84.56% <64.81%> (-9.88%)` | :arrow_down: | | [...pi-generator/lib/src/openapi\_generator\_runner.dart](https://app.codecov.io/gh/gibahjoe/openapi-generator-dart/pull/98?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-b3BlbmFwaS1nZW5lcmF0b3IvbGliL3NyYy9vcGVuYXBpX2dlbmVyYXRvcl9ydW5uZXIuZGFydA==) | `72.76% <73.33%> (-1.15%)` | :arrow_down: | | [...-generator/lib/src/models/generator\_arguments.dart](https://app.codecov.io/gh/gibahjoe/openapi-generator-dart/pull/98?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-b3BlbmFwaS1nZW5lcmF0b3IvbGliL3NyYy9tb2RlbHMvZ2VuZXJhdG9yX2FyZ3VtZW50cy5kYXJ0) | `93.50% <78.57%> (-3.47%)` | :arrow_down: | | [...api-generator/lib/src/extensions/type\_methods.dart](https://app.codecov.io/gh/gibahjoe/openapi-generator-dart/pull/98?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-b3BlbmFwaS1nZW5lcmF0b3IvbGliL3NyYy9leHRlbnNpb25zL3R5cGVfbWV0aG9kcy5kYXJ0) | `67.50% <80.55%> (+10.60%)` | :arrow_up: | | [openapi-generator/lib/src/gen\_on\_spec\_changes.dart](https://app.codecov.io/gh/gibahjoe/openapi-generator-dart/pull/98?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-b3BlbmFwaS1nZW5lcmF0b3IvbGliL3NyYy9nZW5fb25fc3BlY19jaGFuZ2VzLmRhcnQ=) | `90.90% <85.71%> (-2.43%)` | :arrow_down: |

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