[MAINT] refactor github_repository_file to use Context-aware provider functions (#3107)

* Refactor `github_repository_file` to use Context-aware functions for easier debugging

Signed-off-by: Timo Sand <[email protected]>

* Extract importer to separate function

Signed-off-by: Timo Sand <[email protected]>

* Add top-level `Description` field

Signed-off-by: Timo Sand <[email protected]>

* Cleanup references/dereferences with `github.Ptr`

Signed-off-by: Timo Sand <[email protected]>

* Refactor to use `tflog` instead of `log`

Signed-off-by: Timo Sand <[email protected]>

* Refactored the createBranch functionality into a function

Signed-off-by: Timo Sand <[email protected]>

* Replace conditions with validation

Signed-off-by: Timo Sand <[email protected]>

* Minor cleanup

Signed-off-by: Timo Sand <[email protected]>

* Address review comments

Signed-off-by: Timo Sand <[email protected]>

* Address review comments

Signed-off-by: Timo Sand <[email protected]>

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Signed-off-by: Timo Sand <[email protected]>

Author: deiga