chore(.gemini): add config

[noahdietz]

Proposal to further configure gemini assistance on the repo in the following ways:

• disable have_fun to keep feedback concise
• disable posting the help message, it's not particularly useful when we have public docs
• run code review and include draft PRs
• (the big one) attempt to limit the scope of review, excluding what I think would be GAPIC generate client libraries

Context: Gemini posted comments to auto-generated code in #16998, which we will not address, and I'd like to reduce noise / save some compute.

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[parthea]

LGTM as an initial configuration. There are many more files that we should add to ignore_patterns but we can continue to build on this in subsequent PRs.