feat(github client): add functionality for reading a github release by tag

[jan-zabloudil]

When user creates a new release, **existing*** git tag must be provided in order to link release to source code. Once these PR are merged, release service will:

• check if provided git tag actually exists (https://github.com/jan-zabloudil/release-manager/pull/87)
• check if provided git tag is associated with github release (this PR)
• create a github release (if tag is not associated with one yet) and user allows github release creation (https://github.com/jan-zabloudil/release-manager/pull/86)

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*) In the first MVP version, only existing git tags will be supported. Later, the functionality will be extended to allow the use of new git tags as well (ReleaseManager would create a new git tag).