ci: add quarterly Dependabot scan for cargo and github-actions

[soloking1412]

High Level Overview of Change

Adds .github/dependabot.yml, configuring a quarterly Dependabot scan for both dependency surfaces in this repo: cargo (the crate's own dependencies, declared in Cargo.toml) and github-actions (the workflow files under .github/workflows/).

Context of Change

Requested in #295, which points to xrpl-py's existing Dependabot config as the pattern to follow (interval: quarterly, time: "15:00", open-pull-requests-limit: 30) — xrpl-py uses pip as its ecosystem; the Rust equivalent is cargo. github-actions is added alongside it since this repo has its own CI workflows with pinned action versions, and Dependabot is the mechanism that keeps those bump PRs flowing automatically.

Type of Change

• New feature (CI/dependency hygiene)

Before / After

Before: no .github/dependabot.yml — dependency and Action-version updates are entirely manual.

After:

version: 2
updates:
  - package-ecosystem: cargo
    directory: "/"
    schedule:
      interval: quarterly
      time: "15:00"
    open-pull-requests-limit: 30
  - package-ecosystem: github-actions
    directory: "/"
    schedule:
      interval: quarterly
      time: "15:00"
    open-pull-requests-limit: 10

open-pull-requests-limit is lower for github-actions (10 vs. 30) since this repo has only four workflow files and far fewer action references than crate dependencies.

Test Plan

Validated the YAML parses and matches the Dependabot configuration schema (version: 2, valid package-ecosystem values, correctly nested schedule). This is a config-only change — no code, no cargo test impact.

Closes #295