feat: add Branches query type for listing repository branches

[sebagarayco]

Summary

• Adds a new Branches query type to the GitHub datasource plugin, allowing users to list all branches of a repository
• Supports an optional Filter field (e.g. release/) that narrows results to branches whose names match the given prefix, covering the release/* use case
• Works as a Grafana template variable: set Field Value = name to use branch names as variable options in dashboards
• Related issues: Feature request: Add ability to filter by branch on Workflow Usage stats #310, Add ability to get information about the branches of the repository #94

Implementation

Backend (Go):

• pkg/models/branches.go — ListBranchesOptions with owner, repository, and query filter
• pkg/github/branches.go — GraphQL query against refs(refPrefix: "refs/heads/") with pagination, mirrors the existing Tags implementation
• pkg/github/branches_handler.go + datasource.go + query_handler.go — handler wired into the query mux

Frontend (TypeScript/React):

• src/views/QueryEditorBranches.tsx — editor with an optional Filter input (placeholder: release/)
• src/types/query.ts / src/constants.ts — BranchesOptions type and 'Branches' registered in the query type list

Test Plan

• Go unit test passes: go test ./pkg/github/... -run TestListBranches
• All Go tests pass: go test ./...
• All frontend tests pass: npm test
• In Grafana: create a Query variable using the GitHub datasource, select Branches, set owner/repo — branch names appear as variable options
• Enter release/ in the Filter field — results narrow to matching branches only

[cla-assistant]

All committers have signed the CLA.

[sebagarayco]

Hi @alyssajoyner ! any chance to get this reviewed?
Thanks!