Add support for custom buildkite pipelines for public events

[pgarg66]

Solana will host hackathon events, where attendees may want to try their code change to break/hack Solana's testnet. We need a mechanism to submit their PRs to a build system where it runs against a testnet instance.

This testnet instance should not overlap with CI pipelines.

[pgarg66]

Heroku app, influxDB, and buildkite will require some manual configuration.

1. Define a newlabel that can be applied to PRs (or use an existing one, that's not being used)

Heroku

1. Add to CUSTOM_PR_LABEL_BUILDS
2. Add "solana-newlabel" to BUILDKITE_PIPELINE_PUBLIC_LOG_WHITELIST

InfluxDB

1. Configure the database for the new label. Use testnet-newlabel as the database tag
2. Provide read/write access to "scratch_writer" user

buildkite

1. Add a new pipeline to buildkite (clone testnet-automation)
2. Copy env variables from testnet-automation
3. Define the following environment variables

• CLIENT_COUNT=1
• LEADER_CPU_MACHINE_TYPE="n1-standard-16"
• TESTNET_TAG=testnet-newlabel
• INFLUX_DATABASE="testnet-newlabel"
• SOLANA_METRICS_CONFIG (update this variable to use testnet-newlabel as the db)

4. Following environment variables can be set based on the testnet requirements

• NUMBER_OF_NODES="10 25 50 100"
• TESTNET_ZONE=us-west1-b

[pgarg66]

@aeyakovenko , this can be used for hackathon events. Some config is needed, but no code changes are required.