[Gists] Add tool for retrieving the contents of a specific gist

[lostintangent]

Describe the feature or problem you’d like to solve

The gists toolset includes a tool for listing a user's gists, but it doesn't currently provide a tool for retrieving the contents of a specific gist. Which I think is necessary to complete the E2E of an agent being able to discover and then access gists.

Proposed solution

Add a new tool to the gists toolset that accepts a gist ID and returns the file contents + metadata of the specified gist (e.g. get_gist). The GitHub Gists API already supports this, so it would be a fairly straight forward pass through.

[Vidit-Ostwal]

Hi @lostintangent , @mattdholloway
I have added this PR which should close this feature request
#1105