build(dataflow): Add ktlint plugin in Gradle, run formatter, and fix lint issues

[KengoA]

What this PR does / why we need it:

Currently the dataflow engine has no form of linting or formatting whatsoever. This PR:

• introduces ktlint in the Gradle wrapper through https://github.com/JLLeitschuh/ktlint-gradle and sets up minimal configuration for linting
• sets up corrersponding make commands make lint, make format and make build
• runs make format to fix issues automatically
• manually modifies code to fix lint issues
  • Following no-wild-imports rule (https://pinterest.github.io/ktlint/1.2.1/rules/standard/#no-wildcard-imports)
  • Following backing-property-naming rule (https://pinterest.github.io/ktlint/1.2.1/rules/experimental/#property-naming)
  • Fix inline comments (discouraged-comment-location rule) https://pinterest.github.io/ktlint/1.2.1/rules/experimental/#discouraged-comment-location
  • Exceeded max line length (140) issues

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[sakoush]

Lgtm,

Is linting running in the CI as well? If not please add it.

[KengoA]

Is linting running in the CI as well?

Yes I added an extra lint step here:

seldon-core/.github/workflows/lint.yml

Line 40 in cc5d748

- name: lint-dataflow

Runs the worfklow here successfully: https://github.com/SeldonIO/seldon-core/actions/runs/8543232746/job/23406587633?pr=5491