Added support for retrieving specific file content in the github_repository_content table Closes #444

[ParthaI]

Example query results

Results

> select type, name, path, path_raw,mode, line_count from github_repository_content where repository_full_name = 'turbot/steampipe-plugin-github' and path = 'docs/tables/github_audit_log.md'
+------+---------------------+---------------------------------+----------------------------------------------+------+------------+
| type | name                | path                            | path_raw                                     | mode | line_count |
+------+---------------------+---------------------------------+----------------------------------------------+------+------------+
| blob | github_audit_log.md | docs/tables/github_audit_log.md | ZG9jcy90YWJsZXMvZ2l0aHViX2F1ZGl0X2xvZy5tZA== | 0    | 204        |
+------+---------------------+---------------------------------+----------------------------------------------+------+------------+

In retrospection, the code requires some more updates.

[misraved]

@ParthaI the implementation does not look correct since the two columns mentioned in the comment below are not returned by the GetConfig.

Reference - https://github.com/turbot/steampipe-plugin-github/pull/445/files#diff-b859c63e62f546ae6b1c4203d02d7866bc5172eb74f7a57d201aa21c74a705c1R140

Will it be possible to raise an issue with the GitHub API/Graphql team to add the following two fields in the response?

[ParthaI]

@misraved, I have pushed a commit to consistently populate the is_generated and mode column values.

Commit reference: 881047a

Could you please review the changes and let me know if everything looks good?

Thanks!